nips nips2013 nips2013-73 knowledge-graph by maker-knowledge-mining
Source: pdf
Author: Fajwel Fogel, Rodolphe Jenatton, Francis Bach, Alexandre D'Aspremont
Abstract: Seriation seeks to reconstruct a linear order between variables using unsorted similarity information. It has direct applications in archeology and shotgun gene sequencing for example. We prove the equivalence between the seriation and the combinatorial 2-SUM problem (a quadratic minimization problem over permutations) over a class of similarity matrices. The seriation problem can be solved exactly by a spectral algorithm in the noiseless case and we produce a convex relaxation for the 2-SUM problem to improve the robustness of solutions in a noisy setting. This relaxation also allows us to impose additional structural constraints on the solution, to solve semi-supervised seriation problems. We present numerical experiments on archeological data, Markov chains and gene sequences. 1
Reference: text
sentIndex sentText sentNum sentScore
1 It has direct applications in archeology and shotgun gene sequencing for example. [sent-23, score-0.317]
2 We prove the equivalence between the seriation and the combinatorial 2-SUM problem (a quadratic minimization problem over permutations) over a class of similarity matrices. [sent-24, score-0.782]
3 The seriation problem can be solved exactly by a spectral algorithm in the noiseless case and we produce a convex relaxation for the 2-SUM problem to improve the robustness of solutions in a noisy setting. [sent-25, score-0.934]
4 This relaxation also allows us to impose additional structural constraints on the solution, to solve semi-supervised seriation problems. [sent-26, score-0.725]
5 The seriation problem seeks to reconstruct this linear ordering based on unsorted, possibly noisy, similarity information. [sent-30, score-0.758]
6 envelope reduction algorithms for sparse linear algebra [2], in identifying interval graphs for scheduling [3], or in shotgun DNA sequencing where a single strand of genetic material is reconstructed from many cloned shorter reads, i. [sent-34, score-0.312]
7 With shotgun gene sequencing applications in mind, many references focused on the Consecutive Ones Problem (C1P) which seeks to permute the rows of a binary matrix so that all the ones in each column are contiguous. [sent-37, score-0.413]
8 In particular, [3] studied further connections to interval graphs and [6] crucially showed that a solution to C1P can be obtained by solving the seriation problem on the squared data matrix. [sent-38, score-0.651]
9 On the algorithmic front, the seriation problem was shown to be NP-Complete by [10]. [sent-40, score-0.581]
10 More general ordering problems were studied extensively in operations research, mostly in connection with the Quadratic Assignment Problem (QAP), for which several convex relaxations were studied in e. [sent-43, score-0.291]
11 Since a matrix is a permutation matrix if and only if it is both orthogonal and 1 doubly stochastic, much work also focused on producing semidefinite relaxations to orthogonality constraints [13, 14]. [sent-46, score-0.828]
12 These programs are convex hence tractable but the relaxations are usually very large and scale poorly. [sent-47, score-0.184]
13 More recently however, [15] produced a spectral algorithm that exactly solves the seriation problem in a noiseless setting, in results that are very similar to those obtained on the interlacing of eigenvectors for Sturm Liouville operators. [sent-48, score-0.807]
14 They show that for similarity matrices computed from serial variables (for which a total order exists), the ordering of the second eigenvector of the Laplacian (a. [sent-49, score-0.292]
15 Here, we show that the solution of the seriation problem explicitly minimizes a quadratic function. [sent-53, score-0.67]
16 While this quadratic problem was mentioned explicitly in [15], no connection was made between the combinatorial and spectral solutions. [sent-54, score-0.263]
17 Our result shows in particular that the 2-SUM minimization problem mentioned in [10], and defined below, is polynomially solvable for matrices coming from serial data. [sent-55, score-0.177]
18 This result allows us to write seriation as a quadratic minimization problem over permutation matrices and we then produce convex relaxations for this last problem. [sent-56, score-1.204]
19 This relaxation appears to be more robust to noise than the spectral or combinatorial techniques in a number of examples. [sent-57, score-0.294]
20 Perhaps more importantly, it allows us to impose additional structural constraints to solve semi-supervised seriation problems. [sent-58, score-0.643]
21 We also develop a fast algorithm for projecting on the set of doubly stochastic matrices, which is of independent interest. [sent-59, score-0.285]
22 In Section 2, we show a decomposition result for similarity matrices formed from the C1P problem. [sent-61, score-0.178]
23 This decomposition allows to make the connection between the seriation and 2-SUM minimization problems on these matrices. [sent-62, score-0.616]
24 In Section 3 we use these results to write convex relaxations of the seriation problem by relaxing permutation matrices as doubly stochastic matrices in the 2-SUM minimization problem. [sent-63, score-1.545]
25 , n} while the notation ⇧ (in capital letter) will refer to the corresponding matrix permutation, which is a {0, 1} matrix suchP ⇧ij = 1 P ⇡(j) = i. [sent-74, score-0.156]
26 For a vector that iff n n 2 y 2 Rn , we write var(y) its variance, with var(y) = i=1 yi /n ( i=1 yi /n)2 , we also write y[u,v] 2 Rv u+1 the vector (yu , . [sent-75, score-0.215]
27 We write Sn the set of symmetric matrices of dimension n, k · kF denotes the Frobenius norm and i (X) the ith eigenvalue (in increasing order) of X. [sent-80, score-0.225]
28 2 Seriation & consecutive ones Given a symmetric, binary matrix A, we will focus on variations of the following 2-SUM combinatorial minimization problem, studied in e. [sent-81, score-0.271]
29 This problem is used for example to reduce the envelope of sparse matrices and is shown in [10, Th. [sent-84, score-0.154]
30 When A has a specific structure, [15] show that a related matrix reordering problem used for seriation can be solved explicitly by a spectral algorithm. [sent-87, score-0.887]
31 However, the results in [15] do not explicitly link spectral ordering and the optimum of (1). [sent-88, score-0.247]
32 For some instances of A related to seriation and consecutive one problems, we show below that the spectral ordering directly minimizes the objective of problem (1). [sent-89, score-0.883]
33 1 Binary matrices Let A 2 Sn and y 2 Rn , we focus on a generalization of the 2-SUM minimization problem Pn minimize f (y⇡ ) , i,j=1 Aij (y⇡(i) y⇡(j) )2 subject to ⇡ 2 P. [sent-92, score-0.208]
34 (2) The main point of this section is to show that if A is the permutation of a similarity matrix formed from serial data, then minimizing (2) recovers the correct variable ordering. [sent-93, score-0.378]
35 1 We say that the matrix A 2 Sn is an R-matrix (or Robinson matrix) iff it is symmetric and satisfies Ai,j Ai,j+1 and Ai+1,j Ai,j in the lower triangle, where 1 j < i n. [sent-96, score-0.168]
36 8) which is an R-matrix (center), and a matrix a aT where a is a unimodal vector (right). [sent-105, score-0.162]
37 pre-P) iff there is a permutation ⇧ such that ⇧A⇧T is an R-matrix (resp. [sent-109, score-0.247]
38 We can write ij Aij (yi yj )2 = y T LA y where LA = diag(A1) A is the Laplacian of matrix A, which is a block matrix equal to (v u + 1) {i=j} 1 for u i, j v. [sent-122, score-0.208]
39 If ⇧ is such that ⇧C⇧T (hence ⇧A⇧T ) is an R-matrix, then the corresponding permutation ⇡ solves the combinatorial minimization problem (2) for A = C 2 . [sent-134, score-0.347]
40 5 show that each CUT term is minimized by a monotonic sequence, but yi = ai+b means here that all monotonic subsets of y of a given length have the same (minimal) variance, attained by ⇧y. [sent-140, score-0.161]
41 We now show that minimizing (2) also recovers the correct ordering for these more general matrix classes. [sent-146, score-0.185]
42 We call a matrix A pre-Q iff there is a permutation ⇧ such that ⇧A is a Q-matrix. [sent-152, score-0.325]
43 Remark that when A, B are {0, 1} matrices and w = 1, min{Aik , Bkj } = Aik Bkj , so the circle product matches the regular matrix product AB T . [sent-159, score-0.185]
44 Let A = C C T , if ⇧ is such that ⇧A⇧T is an R-matrix, then the corresponding permutation ⇡ solves the combinatorial minimization problem (2). [sent-177, score-0.347]
45 10 show that there is a permutation ⇧ such that ⇧(C C T )⇧T is a sum of CUT matrices (hence a R-matrix). [sent-181, score-0.301]
46 This result shows that if A is pre-R and can be written A = C C T with C pre-Q, then the permutation that makes A an R-matrix also solves (2). [sent-186, score-0.273]
47 3 Convex relaxations for permutation problems In the sections that follow, we will use the combinatorial results derived above to produce convex relaxations of optimization problems written over the set of permutation matrices. [sent-190, score-0.797]
48 We first recall the main result from [15] which shows how to reorder pre-R matrices in a noise free setting. [sent-193, score-0.172]
49 The results in [15] provide a polynomial time solution to the R-matrix ordering problem in a noiseless setting. [sent-199, score-0.185]
50 The results in the previous section made the connection between the spectral ordering in [15] and problem (2). [sent-201, score-0.247]
51 In what follows, we will use (2) to produce convex relaxations to matrix ordering problems in a noisy setting. [sent-202, score-0.369]
52 Numerical experiments in Section 4 show that semi-supervised seriation solutions are sometimes significantly more robust to noise than the spectral solutions ordered from the Fiedler vector. [sent-204, score-0.772]
53 We write Dn the set of doubly stochastic matrices in Rn⇥n , i. [sent-206, score-0.444]
54 Classical results show that the set of doubly stochastic matrices is the convex hull of the set of permutation matrices. [sent-210, score-0.65]
55 a matrix is a permutation matrix if and only if it is both doubly stochastic and orthogonal. [sent-213, score-0.635]
56 This means that we can directly write a convex relaxation to the combinatorial problem (2) by replacing P with its convex hull Dn , to get minimize g T ⇧T LA ⇧g subject to ⇧ 2 Dn , (3) where g = (1, . [sent-214, score-0.4]
57 To provide a solution to the combinatorial problem (2), we then generate permutations from the doubly stochastic optimal solution to (3) (we will describe an efficient procedure to do so in §3). [sent-221, score-0.509]
58 The results of Section 2 show that the optimal solution to (2) also solves the seriation problem in the noiseless setting when the matrix A is of the form C C T with C a Q-matrix and y is an affine transform of the vector (1, . [sent-222, score-0.783]
59 As the set of permutation matrices P is the intersection of the set of doubly stochastic matrices D and the set of orthogonal matrices O, i. [sent-233, score-0.833]
60 p As a doubly stochastic matrix of Frobenius norm n is necessarily orthogonal, we would ideally like to solve 1 minimize p Tr(Y T ⇧T LA ⇧Y ) µ k⇧k2 F p (5) subject to eT ⇧g + 1 eT ⇧g, ⇧1 = 1, ⇧T 1 = 1, ⇧ 0, n 1 with µ large enough to guarantee that the global solution is indeed a permutation. [sent-236, score-0.467]
61 The QP relaxation allows us to add convex structural constraints in the problem. [sent-245, score-0.208]
62 In gene sequencing applications, one may want to constrain the distance between two elements (e. [sent-249, score-0.206]
63 mate reads), which would read a ⇡(i) ⇡(j) b and introduce an affine inequality on the variable ⇧ in the QP relaxation of the form a eT ⇧g eT ⇧g b. [sent-251, score-0.192]
64 More generally, we can rewrite problem (6) with nc additional linear constraints as follows 1 minimize p Tr(Y T ⇧T LA ⇧Y ) µ kP ⇧k2 F p (7) subject to DT ⇧g + 0, ⇧1 = 1, ⇧T 1 = 1, ⇧ 0, where D is a matrix of size n ⇥ nc and is a vector of size nc . [sent-253, score-0.264]
65 This procedure is based on the fact that a permutation can be defined from a doubly stochastic matrix D by the order induced on a monotonic vector. [sent-256, score-0.623]
66 To each v, we can associate a permutation ⇧ such that ⇧Dv is monotonically increasing. [sent-258, score-0.194]
67 If D is a permutation matrix, then the permutation ⇧ generated by this procedure will be constant, if D is a doubly stochastic matrix but not a permutation, it might fluctuate. [sent-259, score-0.751]
68 Starting from a solution D to problem (6), we can use this procedure to generate many permutation matrices ⇧ and we pick the one with lowest cost y T ⇧T LA ⇧y in the combinatorial problem (2). [sent-260, score-0.411]
69 a matrix is a permutation matrix if and only if it is both doubly stochastic and orthogonal. [sent-265, score-0.635]
70 Semidefinite relaxations to orthogonality constraints have been developed in e. [sent-267, score-0.2]
71 The convex relaxation in (7) is a quadratic program in the variable ⇧ 2 Rn⇥n , which has dimension n2 . [sent-272, score-0.197]
72 Furthermore, most pre-R matrices formed by squaring pre-Q matrices are very sparse, which considerably speeds up linear algebra. [sent-274, score-0.282]
73 Solving (7) using the conditional gradient algorithm in [18] requires minimizing an affine function over the set of doubly stochastic matrices at each iteration. [sent-280, score-0.392]
74 On the other hand, solving (7) using accelerated gradient algorithms requires solving a projection step on doubly stochastic matrices at each iteration [20]. [sent-282, score-0.456]
75 Given some matrix ⇧0 , the projection problem is written minimize 1 k⇧ ⇧0 k2 F 2 subject to DT ⇧g + 0, ⇧1 = 1, ⇧T 1 = 1, ⇧ 6 0 (8) in the variable ⇧ 2 Rn⇥n , with parameter g 2 Rn . [sent-284, score-0.177]
76 We reorder the rows of the Hodson’s Munsingen dataset (as provided by [22] and manually ordered by [6]), to date 59 graves from 70 recovered artifact types (graves from similar periods containing similar artifacts). [sent-292, score-0.151]
77 Note that the semi-supervised solution actually improves on both Kendall’s manual solution and on the spectral ordering. [sent-324, score-0.216]
78 The mutual information matrix of these variables must be decreasing with |i j| when ordered according to the true generating Markov chain [23, Th. [sent-327, score-0.158]
79 We can thus recover the order of the Markov chain by solving the seriation problem on this matrix. [sent-331, score-0.642]
80 In next generation shotgun gene sequencing experiments, genes are cloned about ten to a hundred times before being decomposed into very small subsequences called “reads”, each fifty to a few hundreds base pairs long. [sent-336, score-0.347]
81 We generate artificial sequencing data by (uniformly) sampling reads from chromosome 22 of the human genome from NCBI, then store k-mer hit versus read in a binary matrix (a k-mer is a fixed sequence of k base pairs). [sent-338, score-0.526]
82 If the reads are ordered correctly, this matrix should be C1P, hence we solve the C1P problem on the {0, 1}-matrix whose rows correspond to k-mers hits for each read, i. [sent-339, score-0.314]
83 the element (i, j) of the matrix is equal to one if k-mer j is included in read i. [sent-341, score-0.155]
84 02 Table 2: Kendall’s ⌧ between the true Markov chain ordering, the Fiedler vector, the seriation QP in (6) and the semi-supervised seriation QP in (7) with varying numbers of pairwise orders specified. [sent-381, score-1.191]
85 We observe the (randomly ordered) model covariance matrix (no noise), the sample covariance matrix with enough samples so the error is smaller than half of the spectral gap, then a sample covariance computed using much fewer samples so the spectral perturbation condition fails. [sent-382, score-0.436]
86 In practice, besides sequencing errors (handled relatively well by the high coverage of the reads), there are often repeats in long genomes. [sent-385, score-0.157]
87 reads that are known to be separated by a given number of base pairs in the original genome. [sent-389, score-0.215]
88 While our algorithm for solving (7) only scales up to a few thousands base pairs on a regular desktop, it can be used to solve the sequencing problem hierarchically, i. [sent-391, score-0.185]
89 Graph connectivity issues can be solved directly using spectral information. [sent-394, score-0.167]
90 Figure 2: We plot the reads ⇥ reads matrix measuring the number of common k-mers between read pairs, reordered according to the spectral ordering on two regions (two plots on the left), then the Fiedler and Fiedler+QP read orderings versus true ordering (two plots on the right). [sent-395, score-1.016]
91 In Figure 2, the two first plots show the result of spectral ordering on simulated reads from human chromosome 22. [sent-397, score-0.465]
92 The full R matrix formed by squaring the reads ⇥ kmers matrix is too large to be plotted in MATLAB and we zoom in on two diagonal block submatrices. [sent-398, score-0.409]
93 In the first one, the reordering is good and the matrix has very low bandwidth, the corresponding gene segment (or contig. [sent-399, score-0.222]
94 In the second the reordering is less reliable, and the bandwidth is larger, the reconstructed gene segment contains errors. [sent-401, score-0.175]
95 The last two plots show recovered read position versus true read position for the Fiedler vector and the Fiedler vector followed by semisupervised seriation, where the QP relaxation is applied to the reads assembled by the spectral solution, on 250 000 reads generated in our experiments. [sent-402, score-0.746]
96 We see that the number of misplaced reads significantly decreases in the semi-supervised seriation solution. [sent-403, score-0.806]
97 A spectral algorithm for envelope reduction of sparse matrices. [sent-413, score-0.187]
98 An analysis of spectral envelope reduction via quadratic assignment problems. [sent-442, score-0.272]
99 Sums of random symmetric matrices and quadratic optimization under orthogonality constraints. [sent-452, score-0.245]
100 A spectral algorithm for seriation and the consecutive ones problem. [sent-463, score-0.807]
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