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MATROID SECRETARIES 3 Minors Project’ THE MATROID SECRETARY PROBLEM FOR MINOR-CLOSED CLASSES AND RANDOM MATROIDS TONY HUYNH AND PETER NELSON Abstract. We prove that for every proper minor-closed class of Fp-representable matroids, there exists an O(1)-competitive algorithm for the matroidM secretary problem on . This result relies on the extremely powerful matroid minor structure theory beingM developed by Geelen, Gerards and Whittle. We also note that, for asymptotically almost all matroids, the matroid secretary algorithm that selects a random basis, ignoring weights, is (2 + o(1))-competitive. In fact, assuming the conjecture that almost all matroids are paving, there is a (1+o(1))- competitive algorithm for almost all matroids. 1. Introduction The secretary problem [25, 8, 10] is a classic online selection problem. The setup is as follows. A set of n ‘secretaries’ is presented to us in a uniformly random order. When a secretary is presented to us, we learn his weight (a non-negative real number) and we must make an irrevocable decision to hire him or not. Once a secretary is hired, the process ends. The goal is to design an algorithm which performs well in expectation. 1 It is well-known that the algorithm that samples and rejects the first e (here, e is the base of the natural logarithm) of the secretaries and then hires the first secretary that is better than all secretaries in the sample will hire the best secretary with probability 1 e . This is asymptotically the best possible [8]. The matroid secretary problem is a very widely studied generalization of the secretary problem, first introduced by Babaioff, Immorlica, and Kleinberg [2]. Here we are given a matroid M whose elements are ‘secretaries’ with an (unknown) weight function w : E(M) R>0. The secretaries are again presented to us online in a random order. When a secretary→ x is presented to us, we learn its weight w(x). At this point, we must make an irrevocable decision to either hire x or not. The weight of a set of secretaries is the sum of their weights. Our goal is to design an algorithm that hires a set of arXiv:1603.06822v5 [math.CO] 2 Oct 2019 secretaries with large weight, subject to the constraint that the hired set of secretaries is an independent set in M. The expected value of an algorithm on (M,w) is the average weight of an independent set it produces, taken over all orderings of E(M) and over any internal randomness of the algorithm. We write OPT(M,w) for the maximum weight of an independent set of M. For c > 1, an algorithm is c-competitive for M if the expected value it outputs for (T. Huynh) Department of Mathematics, Universite´ Libre de Bruxelles, Belgium (P. Nelson) Department of Combinatorics and Optimization, University of Waterloo, Canada E-mail addresses: [email protected], [email protected]. Tony Huynh is supported by ERC grant FOREFRONT (grant agreement no. 615640) funded by the European Research Council under the EU’s 7th Framework Programme (FP7/2007-2013). 1 2 T. HUYNH AND P. NELSON 1 R (M,w) is at least c OPT(M,w) for all weight functions w : E(M) >0. The following deep conjecture remains open. → Conjecture 1 (Babaioff, Immorlica, and Kleinberg [2]). For all matroids M, there is a c-competitive matroid secretary algorithm for M, where c is a universal constant independent of M. The original motivation for considering the matroid secretary problem was mechanism design for online auctions. Here the ‘secretaries’ are agents and their weights correspond to the prices they are willing to pay for auctioned items. It turns out that matroid constraints model many real world situations and they unify many previously considered models. For example, the situation in which an auctioneer wants to sell at most k items corresponds to the case that M is a rank-k uniform matroid. This is known as the multiple-choice secretary problem and was essentially completely solved by Kleinberg [22]. For more on the connection to mechanism design, see [1]. Although Conjecture 1 remains open, steady progress has been made in improving the competitive ratio. Babaioff, Immorlica, and Kleinberg [2] gave an O(log r)-competitive algorithm for all matroids M, where r is the rank of M. This was improved to an O(√log r)-competitive algorithm by Chakraborty and Lachish [3]. The state-of-the-art is an O(log log r)-competitive algorithm, first obtained by Lachish [24]. Using different tools, Feldman, Svensson, and Zenklusen [9] give a simpler O(log log r)-competitive algorithm for all matroids. On the other hand, there exist constant-competitive algorithms for restricted classes of matroids. We now give a partial list of such matroid classes. Graphic matroids have a 2e-competitive algorithm due to Korula and P´al [23]. Cographic matroids have a 3e-competitive algorithm due to Soto [32]. Regular matroids and max-flow min-cut matroids both have 9e-competitive algorithms due to Dinitz and Kortsarz [7]. Our main result is a generalization of all these aforementioned constant-competitive algorithms. 1.1. Our Results. For each prime p, we let Fp denote the field with p elements. A class of matroids is minor-closed if M and N a minor of M implies that N is also in . A properM minor-closed class of∈MF -representable matroids is a minor-closed M p class of Fp-representable matroids that is not equal to the class of all Fp-representable matroids. Our main result is that Conjecture 1 holds for every proper minor-closed class of Fp-representable matroids. Note that graphic matroids, cographic matroids, regular matroids, and max-flow min-cut matroids are all examples of proper minor-closed classes of Fp-representable matroids. Here is our main result. Theorem 2. Suppose that Hypothesis 1 holds and let p be a prime. Then for every proper minor-closed class of Fp-representable matroids, there is a constant c( ) such that every matroid in M has a c( )-competitive matroid secretary algorithm.M M M To be forthright with the reader, we stress that Theorem 2 relies on a structural hypothesis communicated to us by Geelen, Gerards, and Whittle, which has not yet appeared in print. This hypothesis is stated as Hypothesis 1. The proof of Hypothesis 1 will stretch to hundreds of pages, and will be a consequence of their decade-plus ‘matroid MATROID SECRETARIES 3 minors project’. 1 This is a body of work generalising Robertson and Seymour’s graph minors structure theorem [30] to matroids representable over a fixed finite field, leading to a solution of Rota’s Conjecture [14]. See [15] for a discussion of the project. In proving Theorem 2, we develop techniques to handle each of the ingredients that appear in the statement of Hypothesis 1. We believe these techniques may be of independent interest. Also, our proof of Theorem 2 is one of the few instances of using deep results from matroid theory in attacking Conjecture 1. To complement our main result, we also show that Conjecture 1 holds for almost all matroids. For each M, let RB denote the matroid secretary algorithm that chooses a uniformly random basis B of M, and selects precisely the elements of B, ignoring weights. Theorem 3. For asymptotically almost all matroids on n elements, the algorithm RB 3 is (2 + √n )-competitive. The veracity of Theorem 3 is not terribly surprising, although many properties which ‘obviously’ hold for almost all matroids are very difficult to establish. For example, the fact that almost all matroids are k-connected (for each fixed k) was only proved recently [29]. Indeed, the proof of Theorem 3 relies on recent breakthrough results of Pendavingh and van der Pol [28, 29]. One can think of Theorem 2 and Theorem 3 as complementary results. Theorem 3 asserts that the ‘typical’ matroid has a constant-competitive algorithm, so we expect counterexamples to Conjecture 1 to come from structured classes of matroids. On the other hand, Theorem 2 says that restricting to proper-minor closed classes of Fp- representable matroids also cannot produce counterexamples to Conjecture 1. Note that for almost all matroids, the algorithm RB has a better competitive ratio than the best algorithm for the classical secretary problem. Indeed, under a widely believed conjecture in asymptotic matroid theory, we note that there is in fact a (1+ o(1))- competitive algorithm for almost all matroids. Theorem 4. If asymptotically almost all matroids are paving, then thereisa (1+o(1))- competitive matroid secretary algorithm for asymptotically almost all matroids. 1.2. Our Methods. Roughly, Hypothesis 1 asserts that every matroid in a proper minor-closed class of Fp-representable matroids admits a tree-like decomposition into pieces that are ‘close’ to being ‘sparse’ or ‘graph-like’. It is quite interesting that many of the ingredients of Hypothesis 1 have already been considered in the literature. For example, it is already known that ‘sparse’ and ‘graph-like’ matroids (definitions will be given later) have constant-competitive algorithms [32]. Moreover, Dinitz and Kortsarz [7] have a very nice framework for obtaining constant-competitive algorithms for matroids decomposable as 1-, 2-, and 3-sums from matroids for which we already have constant-competitive algorithms. In Theorem 14, we extend this framework to allow tree-decompositions of any fixed ‘thickness’. This is a non-trivial extension, and is necessary since Hypothesis 1 requires ‘thickness’ greater than 3. We also prove that with an appropriate definition of ‘closeness’, if a minor-closed class ′ is close to another minor-closed class for which M M 1We are aware of the problematic family of dyadic matroids recently discovered by Grace and van Zwam [16], but they are not counterexamples to Hypothesis 1.
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