The Bratislava Declaration of Young Researchers is something I was involved in recently. Its preparation was inspired by Slovak Presidency of the EU and it was presented on today's informal Council of Ministers responsible for competitiveness (Research). I hope this will have some follow up, as current trend in funding research in EU is in my opinion (and not only my as this declaration shows) going in the wrong direction.

# Category Archives: Uncategorized

# Why should you not use the automatic assignment by EasyChair

Some days ago I have assigned papers to my PC in ESA Track A. I tried to use automatic assignment in EasyChair and this was a rather disappointing experience. Just by running it I understood why some assignments I got as a PC member in previous conferences were so bad - they bad without any good reason. Let me explain what happened this time.

I clicked the automatic assignment button and I got a very unfair assignment. EasyChair wanted to punish 3 of my PC members by giving them 11, 12 and 15 papers from their No list. In EasyChair one can bid Yes, Maybe and No on papers. I found this rather absurd as 15 paper was more than half of the total review load. The question is: Does this poor guy need to get so many Nos in optimal assignment? The short intuitive answer is: most probably no, as there is a lot of freedom in each maximal size matching (by Gallai-Edmonds decomposition).

Hence, in 2 hours I coded my own assignment procedure. I assumed that in the assignment:

- first, I want to maximize the number of papers assigned to Maybes and Yeses,
- second, under the first assumption, I want to maximize the number of papers assigned Yeses.

This can be formalized as a maximum-cost maximum-flow problem:

- connect the source vertex with each PC member with a zero cost edge of capacity equal to review load,
- connect each PC member with each paper with edges of capacity 1 and cost 0 for No, 10000 for Maybe and 10001 for Yes.
- connect each paper with the sink with edge of cost 0 and capacity 3.

I executed it and I got assignment with the same number of Nos - 80 and Maybes -100 as EasyChair. And of course, it was not much more fair than the one given by EasyChair. If you always wondered how EasyChair computes the assignment this is the way. Essentially, there is no reason for this to be fair in any sense. One is optimizing global cost, so it should be clear that locally it can be bad, e.g., some PC members will get bad assignments without any good reason. It can be even worse as one can be getting many Nos due to the execution order of procedure, as there is always a lot of freedom in the optimal matching. It can happen that even if you bid on almost everything you will get all the Nos if all people bid No on the same papers.

There is a solution here - I added third assumption to my procedure:

- zero, no PC member can get more than X Nos, where X is a parameter.

This can be easily incorporated into the max-flow problem. One just needs to split a vertex representing each PC member into three vertices: PC_0, PC_No and PC_NotNo. The maximum-cost maximum-flow problem becomes:

- connect the source vertex with all PC_0 vertices with a zero cost edge of capacity equal to review load,
- connect PC_0 to PC_No with zero cost edge of capacity X,
- connect PC_0 to PC_NotNo with zero cost edge of capacity equal to review load,
- connect PC_No with edges of cost 0 to all papers with No,
- connect PC_NotNo with each paper with edges of capacity 1 and cost 10001 for Maybe and 10001 for Yes.
- connect each paper with sink with edge of cost 0 and capacity 3.

I executed it - the table below shows the number of Maybes in the assignment in dependence on the parameter X.

X | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |

#Maybes | 120 | 116 | 112 | 109 | 106 | 104 | 102 | 100 | 100 |

One striking thing is that there exists an optimal assignment where the poor guy that was getting 15 Nos is getting only 11 Nos. Essentially, there was no reason for giving 15 Nos to this PC member! This probably already happened to many of you ;). I the final assignment I went for X=4 as this actually meant that almost everyone gets 3 Nos and only 2 PC members get 4 Nos. This much fairer assignment costed only 20 Maybes in the quality, what I found a fair cost of being fairer.

# Highlights of Algorithm Conference - Registration is Open

This summer, on June 6-8 in Paris, we will be having a new algorithmic event: Highlights of Algorithms 2016 (HALG 2016) conference.

This conference will be quite unlike the conferences we are all used to. First of all, it will consist mainly of invited talks and tutorials, accompanied by a smaller number of contributed talks and posters. Secondly, there will be no conference proceedings, i.e., presenting work already published at a different venue or journal (or to be submitted there) is absolutely welcome and even encouraged.

The call for submissions of contributed short talks and posters has been just posted online. The program committee is currently in the process of selecting invited speakers (partially based on the nominations from the community).

We already have a number of very exciting speakers committed and we expect to add a few more names shortly.

The intention here is to make HALG a forum for presenting the highlights of some of the most exciting recent developments in algorithms, discussing potential further advances in this area, as well as networking and initiating new collaborations. In a way, we want HALG to be a place one can go to catch up on what are the hottest results/topics

at the moment, as well as what one missed by not being able to attend all the STOC/FOCS/SODA/ESA/ICALP/PODC/ITCS/… conferences that year.

We believe that this kind of venue is very much needed today and will help to make our community stronger and more cohesive. In particular, we hope that the success of the first edition will make HALG become a recurring event. In fact, HALG draws some inspiration from Highlights of Logic, Games and Automata (HLGA) conference series that became very successful in its respective community already. There are some differences in how both conferences are constructed: HALG will concentrate on invited talks with some contributed talks, whereas HLGA was only having short contributed talks so far; but the overall premise is the same and seems to be working very well. (We also heard some rumors that the next edition of HLGA will move in the direction of HALG and have invited talks too.)

In any case, the HALG 2016 in Paris should be a lot of fun. We hope to see many of you there! So please submit your work to HALG.

This year for the first time we will have a new algorithmic event taking place - Highlights of Algorithms 2016 (HALG 2016) will be in Paris, June 6-8, 2016. I actually hope that this will become a recurring event in European CS calendar. Some crazy idea for future could be to collocate it with Highlights of Logic, Games and Automata conference that inspired a lot creation of HALG. There are, however, differences in how both conferences are organized. HALG will concentrate on invited talks with some contributed talks, whereas the logic Highlights was only having short contributed talks. I heard some rumors that the next edition of Highlights of Logic, Games and Automata will move into the direction of HALG and have invited talks. Anyway, I hope you will plan to attend the event as it already seem that we will have and interesting program with many highlights of recent developments in algorithms.

The HALG conference has opened the registration and the program is available as well - see the links below. The program looks very interesting so I hope many of you will be coming.

Highlights of Algorithms - HALG 2016

June 6-8, 2016, Paris, France

http://highlightsofalgorithms.

The Highlights of Algorithms conference is designed to be a forum for presenting the highlights of recent developments in algorithms and for discussing potential further advances in this area. The conference will provide a broad picture of the latest research in algorithms through a series of survey and invited talks, as well as possibility for all researchers and students to present their recent results through a series of short talks and poster presentations. Attending the Highlights of Algorithms conference will also be an opportunity for networking and meeting leading researchers in algorithms.

The registration for the Highlights of Algorithms conference is open.

To register, please visit the link at http://highlightsofalgorithms.

A preliminary program is available online at http://highlightsofalgorithms.

The program is packed with 28 invited talks and with even a larger number of short contributions.

Those interested in attending the conference are advised to book accommodation as early as possible (given the high hotel prices in Paris).

# ESA 2016 - call for papers

*Guest post by Aleksander Mądry*

This summer, on June 6-8 in Paris, we will be having a new algorithmic event: Highlights of Algorithms 2016 (HALG 2016) conference.

This conference will be quite unlike the conferences we are all used to. First of all, it will consist mainly of invited talks and tutorials, accompanied by a smaller number of contributed talks and posters. Secondly, there will be no conference proceedings, i.e., presenting work already published at a different venue or journal (or to be submitted there) is absolutely welcome and even encouraged.

The call for submissions of contributed short talks and posters has been just posted online. The program committee is currently in the process of selecting invited speakers (partially based on the nominations from the community).

We already have a number of very exciting speakers committed and we expect to add a few more names shortly.

The intention here is to make HALG a forum for presenting the highlights of some of the most exciting recent developments in algorithms, discussing potential further advances in this area, as well as networking and initiating new collaborations. In a way, we want HALG to be a place one can go to catch up on what are the hottest results/topics

at the moment, as well as what one missed by not being able to attend all the STOC/FOCS/SODA/ESA/ICALP/PODC/ITCS/… conferences that year.

We believe that this kind of venue is very much needed today and will help to make our community stronger and more cohesive. In particular, we hope that the success of the first edition will make HALG become a recurring event. In fact, HALG draws some inspiration from Highlights of Logic, Games and Automata (HLGA) conference series that became very successful in its respective community already. There are some differences in how both conferences are constructed: HALG will concentrate on invited talks with some contributed talks, whereas HLGA was only having short contributed talks so far; but the overall premise is the same and seems to be working very well. (We also heard some rumors that the next edition of HLGA will move in the direction of HALG and have invited talks too.)

In any case, the HALG 2016 in Paris should be a lot of fun. We hope to see many of you there! So please submit your work to HALG.

This year for the first time we will have a new algorithmic event taking place - Highlights of Algorithms 2016 (HALG 2016) will be in Paris, June 6-8, 2016. I actually hope that this will become a recurring event in European CS calendar. Some crazy idea for future could be to collocate it with Highlights of Logic, Games and Automata conference that inspired a lot creation of HALG. There are, however, differences in how both conferences are organized. HALG will concentrate on invited talks with some contributed talks, whereas the logic Highlights was only having short contributed talks. I heard some rumors that the next edition of Highlights of Logic, Games and Automata will move into the direction of HALG and have invited talks. Anyway, I hope you will plan to attend the event as it already seem that we will have and interesting program with many highlights of recent developments in algorithms.

The HALG conference has opened the registration and the program is available as well - see the links below. The program looks very interesting so I hope many of you will be coming.

Highlights of Algorithms - HALG 2016

June 6-8, 2016, Paris, France

http://highlightsofalgorithms.

The Highlights of Algorithms conference is designed to be a forum for presenting the highlights of recent developments in algorithms and for discussing potential further advances in this area. The conference will provide a broad picture of the latest research in algorithms through a series of survey and invited talks, as well as possibility for all researchers and students to present their recent results through a series of short talks and poster presentations. Attending the Highlights of Algorithms conference will also be an opportunity for networking and meeting leading researchers in algorithms.

The registration for the Highlights of Algorithms conference is open.

To register, please visit the link at http://highlightsofalgorithms.

A preliminary program is available online at http://highlightsofalgorithms.

The program is packed with 28 invited talks and with even a larger number of short contributions.

Those interested in attending the conference are advised to book accommodation as early as possible (given the high hotel prices in Paris).

*Guest post by Aleksander Mądry*

This summer, on June 6-8 in Paris, we will be having a new algorithmic event: Highlights of Algorithms 2016 (HALG 2016) conference.

This conference will be quite unlike the conferences we are all used to. First of all, it will consist mainly of invited talks and tutorials, accompanied by a smaller number of contributed talks and posters. Secondly, there will be no conference proceedings, i.e., presenting work already published at a different venue or journal (or to be submitted there) is absolutely welcome and even encouraged.

The call for submissions of contributed short talks and posters has been just posted online. The program committee is currently in the process of selecting invited speakers (partially based on the nominations from the community).

We already have a number of very exciting speakers committed and we expect to add a few more names shortly.

The intention here is to make HALG a forum for presenting the highlights of some of the most exciting recent developments in algorithms, discussing potential further advances in this area, as well as networking and initiating new collaborations. In a way, we want HALG to be a place one can go to catch up on what are the hottest results/topics

at the moment, as well as what one missed by not being able to attend all the STOC/FOCS/SODA/ESA/ICALP/PODC/ITCS/… conferences that year.

We believe that this kind of venue is very much needed today and will help to make our community stronger and more cohesive. In particular, we hope that the success of the first edition will make HALG become a recurring event. In fact, HALG draws some inspiration from Highlights of Logic, Games and Automata (HLGA) conference series that became very successful in its respective community already. There are some differences in how both conferences are constructed: HALG will concentrate on invited talks with some contributed talks, whereas HLGA was only having short contributed talks so far; but the overall premise is the same and seems to be working very well. (We also heard some rumors that the next edition of HLGA will move in the direction of HALG and have invited talks too.)

In any case, the HALG 2016 in Paris should be a lot of fun. We hope to see many of you there! So please submit your work to HALG.

The ESA call for papers is out: http://conferences.au.dk/algo16/esa/. The submission deadline is April 21, 23:59 AoE, 2016, whereas the notifications will be send out no later than on June 9, 2016. I hope there will be many submissions to keep us in the PC busy during this time. Anyway, if you plan to submit something, you might read (or maybe read again) the post by Boaz Barak: http://windowsontheory.org/2014/02/09/advice-for-focs-authors/. It contains advice on how to increase your chances that your paper gets accepted to a conference - FOCS in this case. The same applies to most conferences and "putting the worst foot forward" is often the hardest part. However, believe me that doing the opposite usually hurts even more.

# Highlights of Algorithm Conference

*Guest post by Aleksander Mądry*

at the moment, as well as what one missed by not being able to attend all the STOC/FOCS/SODA/ESA/ICALP/PODC/ITCS/… conferences that year.

In any case, the HALG 2016 in Paris should be a lot of fun. We hope to see many of you there! So please submit your work to HALG.

# Two ERC grants in our group are starting!

We are quite happy that this year two ERC grants will be starting in our group. First of all, Marek Cygan very recently got news that his ERC Starting Grant received funding. His grant is on FPT, approximation and metaheuristics. Secondly, our ERC Proof of Concept grant, that is a follow up to my ERC StG grant is starting. This grant aims to commercialise some ideas from algorithmic market modeling and to support formation of a spin-off.

# On a (Somewhat Old) Dynamic Steiner Tree Problem

The dynamic Steiner tree problem has been around for a while already (for 24 years), but it did not get a satisfying answer from efficiency point of view, i.e., one would like to have a fast algorithm that maintains a constant approximate solution and allows to update the set of terminal vertices. On one hand, there have been some studies of the online version of this problem, where we focus on minimizing the number of changes to the tree that are necessary to maintain a good approximation. This line of research was started in the pioneering paper by Imase and Waxman [1] and was later continued in [2, 3, 4]. However, in the online problem one ignores the problem of efficiently finding these changes to the tree. On the other hand, the problem of maintaining a Steiner tree is also one of the important open problems in the network community [5], where it is often referred to as dynamic multi-cast tree problem. The motivating problem is to maintain a cheap communication network between a set of dynamically changing users. While it has been studied for many years, the research resulted only in several heuristic approaches [6, 7, 8, 9], none of which has been formally proved to be efficient.

In our paper "*The Power of Dynamic Distance Oracles: Efficient Dynamic Algorithms for the Steiner Tree*" we have shown the first sublinear time algorithm which maintains a constant approximate Steiner tree under terminal additions and deletions. We show that we can maintain a -approximate Steiner tree of a general graph in time per terminal addition or removal. Here, denotes the stretch of the metric induced by the graph. For planar graphs we achieve the same running time and the approximation ratio of . Observe that due to the sparsity of real world networks only algorithms with such sublinear complexity could be of some practical importance and could potentially be used to reduce the communication cost of dynamic multicast trees. Our paper aims to be a theoretical proof of concept that from algorithmic complexity perspective such algorithms are indeed possible. However, turning these theoretical ideas into useful algorithms will require much more research. In particular, a natural question here is whether the approximation factor of our algorithms could be considerably improved while keeping the running time sublinear?

Let me note that this might be hard as the approximation factor of for Steiner tree in general graphs comes from using 3-approximate oracles [10], using 2-approximation of the Steiner tree by the MST in the metric closure, and -approximate online MST algorithm. In other words we hit two challenging bounds: in order to improve our approximation factors, one would need either to improve the approximation ratio of the oracles which are believed to be optimal, or devise a framework not based on computing the MST. The second challenge would require to construct simple and fast (e.g., near-linear time) approximation algorithms for Steiner tree that would beat the MST approximation ratio of 2. Constructing such algorithms is yet another challenging open problem.

If you would like to learn more on how it works, you can still attend our talk during the coming STOC in Portland on Monday at 9:00, or have look into the full version of the paper on arXiv.

[1] M. Imase and B. M. Waxman. Dynamic Steiner tree problem. SIAM J. Discrete Math., 4(3):369–384, 1991.

[2] N. Megow, M. Skutella, J. Verschae, and A. Wiese. The power of recourse for online MST and TSP. In ICALP, pages 689–700. 2012.

[3] A. Gu, A. Gupta, and A. Kumar. The power of deferral: maintaining a constant-competitive Steiner tree online. In STOC, pages 525–534, 2013.

[4] A. Gupta and A. Kumar. Online Steiner tree with deletions. In SODA, pages 455–467. SIAM, 2014.

[5] X. Cheng, Y. Li, D.-Z. Du, and H. Ngo. Steiner trees in industry. In D.-Z. Du and P. Pardalos, editors, Handbook of Combinatorial Optimization, pages 193–216. Springer US, 2005.

[6] F. Bauer and A. Varma. ARIES: A rearrangeable inexpensive edge-based on-line Steiner algorithm. IEEE Journal of Selected Areas in Communications, 15:382–397, 1995.

[7] E. Aharoni and R. Cohen. Restricted dynamic steiner trees for scalable multicast in datagram networks. In INFOCOM ’97. Sixteenth Annual Joint Conference of the IEEE Computer and Communications Societies. Driving the Information Revolution., Proceedings IEEE, volume 2, pages 876–883 vol.2, Apr 1997.

[8] S.-P. Hong, H. Lee, and B. H. Park. An efficient multicast routing algorithm for delaysensitive applications with dynamic membership. In INFOCOM ’98. Seventeenth Annual Joint Conference of the IEEE Computer and Communications Societies. Proceedings. IEEE, volume 3, pages 1433–1440 vol.3, Mar 1998.

[9] S. Raghavan, G. Manimaran, C. Siva, and R. Murthy. A rearrangeable algorithm for the construction of delay-constrained dynamic multicast trees. IEEE/ACM Transactions on Networking, 7:514–529, 1999.

[10] M. Thorup and U. Zwick. Approximate distance oracles. J. ACM, 52(1):1–24, 2005.

# Online bipartite matching in offline time

In 1973 Hopcroft and Karp gave a very nice time algorithm computing a maximum matching in unweighted bipartite graphs. This algorithm turned out to be the milestone that is hard to beat. The bipartite maximum matching problem has been studied in many different flavors such as online, approximate, dynamic, randomized or the combination of the above. The HK algorithm was improved for sufficiently dense and sufficiently sparse graphs. Nevertheless, for the general case, despite of the wide interest, little improvement over HK was obtained. This is somewhat intriguing, as no reasonable example certifies that the bound for the running time of HK is in fact tight. The HK algorithm is of offline nature and relies heavily on receiving the whole graph before processing it. On the quest for making some progress on the subject matter we aimed for the dynamic setting. Say the vertices need to be matched as they show up in the graph, so some re-matching is allowed and the maximum matching needs to be maintained at all times. Can one do better than simply finding any augmenting path each time a new vertex appears?

Let us narrow down the dynamic scenario we consider: say only the vertices of one side of the graph come in online, and at the time they show up they reveal all their adjacent edges. This scenario is easily motivated as follows.

There is a set of servers and a set of jobs to be executed on these servers. Every job comes with a list of servers capable of performing the job. Of course it is reasonable to serve as many jobs as possible, hence the maximum job-server matching is highly desired. Having that motivation in mind, an online scenario seems to be the most natural: jobs come in online one at the time, and request to be matched to one of the chosen servers. We care to match the job whenever its possible, so we allow some busy servers to be reallocated.

Now the question is, what is a good way of reallocating the servers, so that the dynamic algorithm can benefit from it? We adopt the following measure: each reallocation involves some reallocating cost. We want to minimize the cost of reallocating the servers.

We associate with each server an attribute called , which states how many times the server has been reallocated in the entire process. The parameter stands for time. We see the entire process of adding jobs as a sequence of turns, in each turn a new job appears with the list of eligible servers. The attribute describes the number of times was reallocated up to turn . These attributes guide us when we search for augmenting paths. We try to avoid servers with high ranks. This should distribute the necessary reallocations more or less evenly among the servers.

In order to follow this approach, a certain greedy strategy comes to mind. When trying to match a new job, choose the augmenting path which minimizes the maximum rank of a server along this path. This strategy has one substantial disadvantage. If any augmenting path that we can choose to match the new job contains a vertex of high rank , then we are allowed to rematch all servers of ranks at most . That is clearly an overhead. We hence define another attribute, , for every job . This attribute says what is the rank of the lowest ranked path from to any unmatched server. When we try to match job , we search for the alternating path along which the tiers of jobs do not increase. We call such a path a tiered path. In other words, a tiered path minimizes the maximal rank on its every suffix. This seems natural: why to re-enter vertices of high rank when we can avoid them.

It turns out that this simple strategy does quite well: the ranks (and hence also the tiers) do not grow above . That means, that each server gets reallocated at most times. This is already interesting. Moreover, if we knew how to efficiently choose the right augmenting paths, the total work would be . We have an algorithm that finds all the augmenting paths in the total time , what matches the offline time of HK.

So let us first take a look on how the bound on the maximum rank is obtained. First of all, we direct the edges of the graph according to the current matching: the matched edges point from servers to jobs, while the unmatched edges point the other way around. Now, directed paths are alternating, so in each turn we need to find a directed path from the new job to a free server and reverse the edges along the path. We first show that the servers of high ranks are far from the unoccupied servers: the directed distance from any server to an unmatched server in turn is at least .

We now look at any set of vertex disjoint directed paths covering all free servers in turn before applying the augmenting path. Note, that there are no outgoing edges from free servers, so the paths end there. The rank of the directed path present in the graph in turn is the maximum rank of a server on it. Let's call the augmenting path applied in turn . We analyze the augmentation process backwards. In turn , before applying , there exists a set of vertex disjoint paths covering free servers, such that:

- every path has its counterpart , where is an injection
- has rank at least as high as , unless 's rank is smaller or equal to one plus the rank of : then the rank of may be one less then the rank of
- there is a path in that is not in the image of and has rank at least the rank of

This observation may be translated as follows. Let us, for every path in , draw a vertical bar of height equal to its rank. Let us now sort the bars in descending order and put them next to each other, as shown in Figure 1 to the left. These bars are the ranks of the paths in turn . When we take a step back to turn , we have another set of bars corresponding to paths from turn , where one additional bar pops out. Moreover, some bars may have decreased by one, but all the bars that decreased are dominated (with respect to height) by the newly added bar. This is shown in Figure 1 to the right. The process ends when we reach turn , and there is bars of height zero.

Now we move to bounding the maximum rank. The maximum rank, say , will appear on some path in some turn . We take a look at set consisting of this single path. There is only one bar of height . In the previous turn, either there is still a bar of the height , or there are two bars of height . Every time the bars decrease, there comes another bar that dominates the decreased bars. Somewhere on the way back to turn there is a set of bars with the sum of heights quadratic in . The bars, however, correspond to vertex disjoint paths, and the heights of the bars are the lower bounds on the lengths of these paths. Hence, there is vertices in the graph and .

The question that remains is whether we are able to efficiently find these paths. The main problem here is that we need augmenting paths where the tiers of jobs along the path do not increase. This is not a good news: the tiers are difficult to maintain upon the reversal of the edges on the entire augmenting path. The idea is to maintain them in a lazy manner. For each job , instead of its actual tier, the algorithm maintains an attribute . Subscript LB stands for lower bound, as we maintain the invariant that . When a new vertex turns up in some turn, is set to . The algorithm repeatedly tries to find (in the directed graph) a tiered (according to the maintained lower bounds for tiers) directed path from to a free server. It executes a search routine from , traversing only the vertices with ranks and 's bounded by . Once a job with is discovered, the upper bound on the ranks and 's of vertices visited next is recursively set to . This goes on until one of the two things happen. It might be that a free server is discovered. Then we found an augmenting path, along which the 's of the vertices are their actual tiers (the path that we found is a witness for that). We reverse the edges along the augmenting path and increase the ranks of the reallocated servers. It might also happen that the search fails. This means, that the algorithm detects a group of vertices whose 's are lower than their actual tiers. The algorithm then rightfully increases the 's associated with these vertices. It continues the search from the point where it failed. The difference is that now it can search further, as it is allowed to visit vertices with higher 's than before. The general idea is that whenever a vertex is visited by the algorithm, either its or its rank is increased. Unfortunately upon every such visit the algorithm may touch all the edges adjacent to the visited vertex. These edges, however, will be touched in total times each. The total time amounts to .

# FPT algorithms and syphilis

Long long time ago, ed actually in 1943, US Army was recruiting a lot of soldiers. Each of the recruits had to be subject of some medical examination, and in particular they were tested against syphilis. However, performing a single test was quite expensive. Then they came up with the following idea. Pick blood samples from a group of soldiers, mix them into a one big sample and perform the test on it. If the test is positive, there is at least one infected soldier in the group. But if it is negative, we know that *all* of the soldiers in the group are healthy and we just saved a lot of tests. It becomes then an interesting problem to devise a method which uses this observation to find the exact group of infected recruits among all the candidates with some nice bound on the number of tests. Without any additional assumptions there is not much you can do (exercise: do you see why?) but in this case we expect that the number of infected candidates is quite small. Then in fact you can save a lot and this story gave rise to the whole area called **group testing**.

Group testing is a rich field and includes a variety of models. Let us focus on the following one. We are given a universe and we want to find a hidden subset of . We can aquire information only by asking queries to the intersection oracle, i.e., for a given subset , answers true if and only if has a nonempty intersection with . Moreover, we can decide which set we query based on previous answers. The goal is to use few queries. There are many algorithms for this problem, but I'm going to describe you my favourite one, called the bisecting algorithm. It dates back to early seventies and is due to Hwang, one of the fathers of combinatorial group testing. As you may expect from the name, it is a generalization of binary search. A simple observation is that once answers false, we can safely discard and otherwise we know that contains at least one element of . So assume that in the algorithm we use a oracle, implemented just as . The algorithm works as in the animation below (choose FitPage in the zoom box and keep clicking the arrow down) :

So, basically, we have a partition of a universe (initially equal ) with the property that every set in the partition contains at least one element of . We examine sets of the partition one by one. Each set is divided into two halves and we query whether we can discard one of them. At some point we query a singleton and then we either discard it or find an element of . I hope it is clear now, but let me paste a pseudocode as well. How many queries do we perform? Let and . Call a query positive if the answer is Yes, negative otherwise. For a negative query we know that there is . Assign this query to . Note that for every there are queried sets assigned to it, because if we consider the queries in the order of asking them, every set is roughly twice smaller than the previous one. So there are negative queries. Every set from a positive query is a half of a set from a (different!) negative query, so the total number of positive queries is bounded by the total number of negative ones. Hence we have queries in total. A slightly more careful analysis (see Lemma 2.1 here) gives . Cool, isn't it? Especially given that we hopefully do not expect a large fraction of infected soldiers...

Great, but **is there a connection of all of that with**** the FPT algorithms from the title**? Yes, there is one. Consider for example the k-PATH problem: given a graph and a number we want to *find* a path of length . The corresponding decision problem is NP-complete, as a generalization of Hamiltonian Path. However, it turns out that when is small, you can solve it pretty fast. Perhaps you know the famous Color Coding algorithm by Alon, Yuster and Zwick which solves it in . However, one can do better: Björklund, Husfeldt, Kaski and Koivisto presented a -time Monte-Carlo algorithm! The only catch is that it *only solves the decision problem*. Indeed, it uses the Schwartz-Zippel lemma and when it answers YES, there is no way to trace back the path from the computation.

Now, let the universe be the edge set of our graph. We want to find one of (possibly many) -subsets of corresponding to -edge paths in our graph and the Björklund et al's algorithm is an *inclusion* oracle, which tells you whether a given set of edges contains one of these subsets. So this is not exactly the same problem as before, but sounds pretty similar... Indeed, again we can implement the oracle, i.e., . So it seems we can use the bisecting algorithm to find a -path with only queries to the decision algorithm! Correct?

Well, not exactly. The problem is the oracle is a Monte Carlo algorithm, more precisely it reports false negatives with probability at most, say, 1/4. Together with Andreas Björklund and Petteri Kaski we showed that a surprisingly simple patch to the bisecting algortihm works pretty nice in the randomized setting. The patch is as follows. Once the bisecting algorithm finishes in the randomized setting, we have a *superset* of a solution. Then, as long as we have more than candidate elements, we pick one by one, in a FIFO manner, and check whether we can discard this single element. We show that the expected number of queries is . (Actually, we conjecture it is optimal, i.e., you have to loose a bit compared to the slightly better in the deterministic setting.) As a consequence, we get a pretty fast *implementation* of finding -paths. For example, a (unique) 14-vertex path is found in a 1000-vertex graph well below one minute on a standard PC. Not bad for an NP-complete problem, I would say. Let me add that the Schwartz-Zippel approach is used in a number of FPT algorithms and in many cases the corresponding search problem can be cast in the inclusion oracle model mentioned above. Examples include k-packing, Steiner cycle, rural postman, graph motif and more.

If you want to learn more, see the paper or slides from my recent ESA talk!

# Efficiency of Random Priority

Matchings are one of the most basic primitives used in the area of mechanism design. Whenever we need to assign items to agents we view it as a matching problem. Of course, physician plenty of variations differing in constraints and objectives can be considered, price but let us look at the simplest variant possible. We need to assign students to dorms. Students have preferences over dorms. We, visit this site the administration, would like to find an allocation that would meet students' preferences as much as possible. We cannot use money in deploying the mechanism. This problem have been considered many times in the literature, but in a setting where preferences of student are given by an ordering over the set of dorms. In this setting it was shown that there exists only one truthful, non-bossy and neutral mechanism. The unique mechanism is called Serial Dictatorship and it works as follows. First, agents are sorted in a fixed order, and then the first agent chooses his favorite item, the next agent chooses his favorite item among remaining items, and so on. Moreover, its randomized variant called Random Serial Dictatorship (RSD), which scans according to a random order, possesses another nice properties of being symmetric and ex post efficient, i.e., it never outputs Pareto dominated outcomes. This mechanism is more often called Random Priority, and hence the title of this entry.

However, one can imagine a setting where the preferences of an agent are not given by an ordering of items, but rather where we are given values telling how much agent values item . In this model we can quantify social welfare of an assignment that a mechanism finds. Here, welfare of an assignment is just the sum . Together with Piotr Sankowski and Qiang Zhang we decided to investigate the problem of comparing the social welfare obtained by RSD with the optimum offline welfare. By optimum offline welfare we mean the maximum weighted matching in the underlying graph. We assume that when the RSD proposes an agent to make a his choice, then he will pick an item he values the most. We also assume that an agent resolves ties randomly. At first sight, it might seem like obtaining any meaningful result on approximation factor of RSD is not possible. Just look at the example in the Figure.

For items each agent has value 0, that's why these edges are not in the Figure. The optimal social welfare is obviously 1. However, the first agent approached by RSD will always pick item , and therefore the social welfare of RSD is . To overcome this problem, we asked two questions. First, what if valuations of agents are always either 0 and 1? Here the hard example does not apply anymore, if assumed that ties are resolved randomly. Second, what if the optimal social welfare is big compared to ? One can see, that in an extreme case with the RSD finds welfare of value at least (an inclusion maximal matching), so it's not bad either.

**0/1 preferences**

In case the preferences are 0 and 1, we can look only at the 1-edges in the underlying graph. Then we are given an unweighted graph and we just want to maximize the size of constructed matching. We stress here, that the random tie-breaking rule is crucial here. That is, we require that when RSD asks an agent that has 0 value for all remaining items, then this agent picks one of the items at random. Without this assumption we can see that the hard example from the figure still works --- suppose each agents value all items 0, but each of them always chooses .

In this model, RSD is a -approximation, and below we show why. Instead of clunky ``agent has value 1 for an item'', we shall say shortly ``agent 1-values an item''. Consider step of the algorithm. Some agents and items are removed from the instance, so let be what remains from optimal solution after steps of RSD. Also, let be the partial matching constructed by RSD after steps, and be its welfare. It can happen that at time , an agent does not 1-value any of remaining items, even though he could have 1-valued some of the items initially. Thus let be the number of agents who 0-value all remaining items. Let be the agent who is at step chosen by RSD to pick an item. With probability agent 1-values at least one item. If so, then the welfare of RSD increases by 1, i.e., Hence [mathbb{E}left[

uleft(RSD^{t+1}

ight)

ight]=mathbb{E}left[

uleft(RSD^{t}

ight)

ight]+1-frac{mathbb{E}left[z^{t}

ight]}{n-t}.] What is the expected number of edges we remove from ? That is, what is the expected loss ? Again, with probability agent picks an item he values 1. Both agent and item may belong to , in which case loses two edges. Otherwise loses one edge. Now suppose that agent 0-values all remaining items, which happens with probability . If is such an agent, then he picks an item at random from all remaining items, so with probability agent picks an item that belongs to . If so, then loses 1, and otherwise, does not lose anything. We have , so , and hence the expected decrease is: [egin{eqnarray*}&&mathbb{E}left[

uleft(OPT^{t}

ight)-

uleft(OPT^{t+1}

ight)

ight] \ leq &&mathbb{E}left[ 2cdotleft(1-frac{z^{t}}{n-t}

ight)+frac{z^{t}}{n-t}cdotfrac{

uleft(OPT^{t}

ight)}{n-t}

ight] \ leq &&mathbb{E}left[ 2cdotleft(1-frac{z^{t}}{n-t}

ight)+frac{z^{t}}{n-t}cdot left(1-frac{z^{t}}{n-t}

ight)

ight] \ leq&&3cdotleft(1-frac{mathbb{E}left[z^{t}

ight]}{n-t}

ight). end{eqnarray*}] Therefore, [egin{align*}mathbb{E}left[

uleft(OPT^{t}

ight)-

uleft(OPT^{t+1}

ight)

ight] &leq 3cdotleft(1-frac{mathbb{E}left[z^{t}

ight]}{n-t}

ight) \ &=3cdotmathbb{E}left[

uleft(RSD^{t+1}

ight)-

uleft(RSD^{t}

ight)

ight],end{align*}] and by summing for from to we conclude that [egin{align*}

uleft(OPT

ight)&=mathbb{E}left[

uleft(OPT^{0}

ight)-

uleft(OPT^{n}

ight)

ight]\ &leq3cdotmathbb{E}left[

uleft(RSD^{n}

ight)-

uleft(RSD^{0}

ight)

ight]=3cdot mathbb{E}left[

uleft(RSD

ight)

ight].end{align*}]

**Connection to online bipartite matching**

As we can see, in the analysis the main problem for RSD are agents who have value 0 for remaining items. If we could assume, that such an agent would reveal the fact that he 0-values remaining items, then we could discard this agent, and the above analysis would give a -approximation. But in this model, we could actually implement a mechanism based on the RANKING algorithm of Karp, Vazirani, Vazirani, and this would give us approximation of . In fact, it's also possible to use the algorithm of Mahdian and Yan (2013) and this would give a factor of .

**Big OPT**

Now let us go back to the model with arbitrary real valuations from interval . Here we want to obtain approximation factor that depends on --- for the ratio should be around , while for it should be constant. This suggests that we should aim in approximation factor of . Let us first show an upper-bound that cannot be achieved by any mechanism. Take any integer , and copy times the instance from the Figure. Let agents 0-value items from different chunks, so that social welfare of any mechanism is the sum of welfares in all chunks. On any chunk no mechanism can get outcome better than , hence no mechanism cannot do better than on the whole instance. Here , and the number of agents is this time. Therefore, asymptotic upper-bound on RSD's welfare is , where is the number of agents. But what is quite interesting, we can prove that RSD is only fraction away from this upper-bound. That is, we show that expected outcome of RSD is at least . However, this we shall not prove here. If you are interested from where this shapely constant of comes, then please have a look at our arXiv report.

**SAGT'14**

It turns out that this natural problem at the same time was also investigated independently by Aris Filos-Ratsikas, Søren Kristoffer Stiil Frederiksen, Jie Zhang. They obtained similar results for the general valuations. Both their and our papers are accepted to SAGT'14, and we will have a joint presentation of the results. So if you are going to the symposium, then please give us a pleasure of attending our talk.