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Staggered extraction for exact matrix multiplication over every field

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Abstract

We prove that the arithmetic exponent of square matrix multiplication over every fixed field satisfies ω < 2.371054886006746. This includes every positive characteristic.

Introduction

For a fixed field F\mathbb{F}, the matrix multiplication exponent ω=ωF\omega= \omega_{\mathbb{F}} is the infimum of the real numbers τ\tau for which two n×nn \times n matrices over F\mathbb{F} can be multiplied using O(nτ+o(1))O(n^{\tau+o(1)}) scalar additions and multiplications. This exponent measures the asymptotic arithmetic cost of a central bilinear operation and of many algorithms built from it. The field is fixed throughout; constants in an arithmetic algorithm may depend on the field and on its chosen finite construction.

Strassen’s seven-product algorithm for 2×22 \times2 matrices gave the subcubic exponent log⁡27\log_2 7 [12]. Subsequent advances used tensors to record bilinear operations: tensor rank counts the products of linear forms needed to realize an operation. Approximate bilinear identities and their conversion to exact algorithms [4], Schönhage’s asymptotic sum inequality [10], and Strassen’s laser method [13] established the main ingredients of this approach. The sum inequality converts a low-rank direct sum of matrix tensors into an exponent bound. The laser method constructs such direct sums by restricting powers of a tensor with low asymptotic rank. Coppersmith and Winograd combined these ideas with progression-free sets and introduced the tensor family used here [5].

Passing to a larger tensor power gives more freedom in choosing the intermediate pieces before extracting matrix tensors from them. Stothers analyzed the fourth power, in work subsequently published with Davie [11, 6]. Vassilevska Williams analyzed the eighth power, and Le Gall continued through the thirty-second power [14, 9]. Alman and Vassilevska Williams then reduced the extraction loss that arises when prescribed marginal distributions permit several joint distributions, obtaining ω<2.3728596\omega< 2.3728596 [2].

A further source of loss is requiring entire variable blocks to be disjoint when the final matrix tensors need only disjoint subsets of those blocks. Duan, Wu, and Zhou used asymmetric hashing to reduce this combination loss in the recursive analysis [7]. Vassilevska Williams, Xu, Xu, and Zhou allowed finer sharing of variables and extended recovery from deletions on all three sides to the restricted tensor products needed in the recursion [15]. Alman, Duan, Vassilevska Williams, Xu, Xu, and Zhou then introduced successive compatibility tests that treat all three sides differently [1]. These tests underlie the extraction studied here.

For a recent numerical comparison, Dupont et al. obtained ω<2.371177\omega< 2.371177 by optimizing the eighth-power instance of this asymmetric framework, combining modern optimization with AlphaEvolve [8]. Version 3 of More Asymmetry Yields Faster Matrix Multiplication records that improvement and credits the new parameters to Dupont et al. [1], Table 1. We prove the following bound over every fixed field.

left and right shapesprobabilityproduct of the two matrix tensors
(0,0,2), (1,1,0)(0,0,2),\ (1,1,0)d/2d/2⟨1,5,1⟩\langle1,5,1\rangle
(1,1,0), (0,0,2)(1,1,0),\ (0,0,2)d/2d/2⟨1,5,1⟩\langle1,5,1\rangle
(0,1,1), (1,0,1)(0,1,1),\ (1,0,1)(1−d)/2(1-d)/2⟨5,1,5⟩\langle5,1,5\rangle
(1,0,1), (0,1,1)(1,0,1),\ (0,1,1)(1−d)/2(1-d)/2⟨5,1,5⟩\langle5,1,5\rangle

Table 1.

Theorem 1.1. Over every fixed field F\mathbb{F}, the arithmetic exponent of square matrix multiplication satisfies

ωF<2.371054886006746<2.371054887.\omega_{\mathbb{F}} < 2.371054886006746 < 2.371054887.

The construction gives exact arithmetic algorithms, including in positive characteristic. Its finite choices are independent of the input matrices. The theorem concerns asymptotic arithmetic cost; the counting arguments and parameter choices do not give bit-complexity or practical crossover estimates.

Proof overview

We use the Coppersmith–Winograd tensor with parameter 5, denoted CW5\mathrm{CW}_5. Section 2 defines it and proves that the rank of its mmth tensor power is at most 7m7^m times a polynomial in mm, over the original field. A strand of length ℓ\ell is a copy of CW5⊗ℓ\mathrm{CW}_5^{\otimes\ell}. On each side the variables of CW5\mathrm{CW}_5 are indexed by 0,…,60,\ldots,6, with weights 0,1,1,1,1,1,20,1,1,1,1,1,2. A strand variable is a word of these indices, and a shape records the weight sums on the three sides. We start with strands of length 8 and split them successively into lengths 4, 2, and 1. At each depth we restrict by shape. A shape with a zero coordinate already gives a matrix multiplication tensor; positive shapes pass to the next split.

Combining capacities across depths. An extraction restricts a tensor to a direct sum of specified tensor products. To make the outputs disjoint, its three side-assignment tests each impose a bound on the guaranteed logarithmic number of outputs; we call these bounds its directional capacities. The guarantee uses their minimum. The constituent-stage analysis in [1] Theorem 6.4 already combines differently prescribed populations at one depth before taking this minimum. Our joint step also combines populations at different depths, provided their priority orders agree on the physical tensor sides. For their weighted capacity vectors vr∈R3\mathbf{v}_r \in\mathbb{R}^3, the benefit of pooling across depths is expressed by

∑rmin⁡Wvr,W≤min⁡W∑rvr,W.\sum_r \min_W v_{r,W} \leq\min_W \sum_r v_{r,W}.

Thus a shortage on one side at one depth can be offset by capacity from another depth. Section 3 proves this joint extraction by following the actual order of the tests. On the first side, variables are grouped by their sequences of strand or half-strand weight sums; these groups are the coarse blocks. Each retained coarse block is assigned to a unique output. Compatibility with that assignment then determines which complete variables on the second and third sides can be retained. The result is a direct sum of the intended products with some variables deleted. Populations retain their ancestry and physical placement throughout, so each subsequent split receives its own prescribed distributions.

Recovering outputs with inherited restrictions. Some variables are missing because an earlier extraction imposed type conditions, meaning conditions on empirical distributions of variable statistics. Others are deleted to resolve competing assignments in the current extraction. Earlier recovery arguments already treat three-side deletions and inherited type conditions [15] Section 7, [1] Section 6.5. Section 4 proves the form needed for our joint step. With NN the large strand-count parameter, the extraction selects outputs in which every orbit of complete variables under the allowed position permutations loses at most C/NC/N of its variables, uniformly over the orbits. Here CC depends only on the fixed population data and tolerances. Independent symmetry shifts then cover the ideal tensor. Partitioning variables by their presence in those shifts reconstructs every coefficient exactly once, over the original field. Only a subexponential number of independent input copies is needed. The recovered products can then be subdivided and used in further extractions.

Staggering and the final rank comparison. A lot contains NN length-8 strands. We stagger finitely many lots so that, at an interior time step, one lot is at each of the three splitting stages. Stage priorities balance the summed directional capacities. The work is partitioned among the six physical axis orders; one joint extraction acts on each part, and the six output counts multiply. Section 5 constructs this schedule, shown in Figure 1, and computes its finite boundary loss. Section 6 counts the common oriented dimensions of the terminal matrix tensors and combines these counts with the schedule to prove the total yield and rank comparison.

Mixed-depth extraction with finite boundaries

Figure 1. Mixed-depth extraction with finite boundaries (K≥2K \ge2). A lot advances one stage per tick, while a full tick combines three different lots. Within each physical-order factor, the common order σ\sigma permits one application of Theorem 3.1 to the summed capacities. The four boundary ticks have the positive minima appearing in (42); they incur only the fixed deficit β\beta. The middle column is absent when K=2K = 2.

Write H0H_0 for the initial logarithmic yield, C∗C_* for the contribution of the three staggered stages, and S∗S_* for the terminal logarithmic matrix volume, all normalized per initial length-8 strand. The direct-sum rank inequality then gives

ωF≤3(8log⁡7−H0−C∗)S∗.(1)\omega_F \le\frac{3(8\log7-H_0-C_*)}{S_*}. \tag*{(1)}

For each fixed number of lots, we first take the divisible strand count NN to infinity and remove approximation losses; the number of lots tends to infinity last. Thus the boundary cost vanishes without a uniform assertion for a growing schedule. The explicit rational data in Section 7 and Appendix A certify S∗>0S_* > 0 and place the ratio in (1) below the bound in Theorem 1.1. The complete parameter verifier and its rational-logarithm interval certificate accompany the source. The proof requires a single feasible choice of parameters, with no assertion that this choice is optimal.

Rank and the basic tensor

Fix an arbitrary field F\mathbb{F}. All tensor identities and linear maps below are over F\mathbb{F}, and all logarithms are natural. A tensor is viewed as a trilinear form in three disjoint sets of variables. Its rank R(T)R(T) is the least number of products of three linear forms, one on each side, whose sum is TT. A restriction substitutes linear forms separately on the three sides. Restrictions cannot increase rank. A direct sum uses disjoint variables, and a tensor product pairs the variable indices on each side; rank is subadditive under direct sum and submultiplicative under tensor product. These statements follow by applying the indicated operations to rank decompositions.

For positive integers a,b,ca,b,c, write

⟨a,b,c⟩=∑i=1a∑j=1b∑k=1cxijyjkzki.\langle a,b,c\rangle=\sum_{i=1}^{a}\sum_{j=1}^{b}\sum_{k=1}^{c}x_{ij}y_{jk}z_{ki}.

This is the tensor for multiplying an a×ba \times b matrix by a b×cb \times c matrix. Its volume is abcabc. Pairing the three matrix indices gives

⟨a,b,c⟩⊗⟨a′,b′,c′⟩≅⟨aa′,bb′,cc′⟩.\langle a,b,c\rangle\otimes\langle a',b',c'\rangle\cong\langle aa',bb',cc'\rangle.

Cyclically permuting the three variable spaces replaces the parameters by (b,c,a)(b,c,a) or (c,a,b)(c,a,b) and preserves rank.

Exact rank and the arithmetic exponent

Put Rd=R(⟨d,d,d⟩)R_d=R(\langle d,d,d\rangle) and

w=inf⁡d≥2log⁡Rdlog⁡d,d an integer.(2)w=\inf_{d\ge2}\frac{\log R_d}{\log d},\qquad d\text{ an integer}. \tag*{(2)}

The flattening that sends a zz coordinate to its coefficient bilinear form has rank d2d^2: the d2d^2 forms ∑jxijyjk\sum_j x_{ij}y_{jk} are linearly independent. A rank-one tensor has flattening rank at most one. Thus d2≤Rd≤d3d^2 \le R_d \le d^3 and 2≤w≤32 \le w \le3. In particular,

Rd≥dw(d≥1),(3)R_d \ge d^w \qquad(d \ge1), \tag*{(3)}

where R1=1R_1=1 handles the unit case.

Let ω\omega denote the arithmetic exponent of square matrix multiplication over F\mathbb{F}, counting scalar additions and multiplications. A fixed rank-RdR_d decomposition gives a bilinear matrix multiplication identity with RdR_d products. This identity remains valid on matrix blocks: every product has its xx-linear factor before its yy-linear factor, and comparison of coefficients proves the same identity for noncommuting blocks with central coefficients in F\mathbb{F}. At size dkd^k the resulting arithmetic cost satisfies

Ck≤RdCk−1+Od(d2k).C_k \le R_d C_{k-1} + O_d(d^{2k}).

The implied constant may depend on the chosen decomposition. Since Rd≥d2R_d \ge d^2, this gives Ck=Od(kRdk)C_k = O_d(kR_d^k), including the boundary case Rd=d2R_d=d^2. Padding an arbitrary size to the next power of dd therefore gives cost Od(nlog⁡dRdlog⁡(n+1))O_d(n^{\log_d R_d}\log(n+1)). The logarithmic factor is absorbed by nηn^\eta for every η>0\eta>0; taking the infimum in (2) proves

ω≤w.(4)\omega\le w. \tag*{(4)}

We also need a uniform upper bound on exact square rank. Given ε>0\varepsilon>0, choose one integer d≥2d\ge2 with θ=log⁡dRd<w+ε\theta=\log_d R_d<w+\varepsilon. For h≥1h\ge1, tensor powers and padding give

R(⟨h,h,h⟩)≤Rd⌈log⁡dh⌉≤Cεhw+ε,Cε=Rd.(5)R(\langle h,h,h\rangle) \le R_d^{\lceil\log_d h\rceil} \le C_\varepsilon h^{w+\varepsilon}, \qquad C_\varepsilon=R_d. \tag*{(5)}

This argument uses one approximating dimension; it does not require the infimum defining ww to be attained.

We use the following identical-summand form of Schönhage’s asymptotic sum inequality [10], with a proof from exact rank.

Lemma 2.1 (Direct sums of identical matrix tensors). Let s,a,b,cs,a,b,c be positive integers. If

R(⨁j=1s⟨a,b,c⟩)≤R∗,R\left(\bigoplus_{j=1}^{s}\langle a,b,c\rangle\right) \le R_*,

then

s(abc)w/3≤R∗.(6)s(abc)^{w/3} \le R_*. \tag*{(6)}

Proof. First consider ss copies of ⟨d,d,d⟩\langle d,d,d\rangle. When s=1s=1, the assertion sdw≤R∗sd^w\le R_* is (3). When d=1d=1, the direct sum is ∑j=1sxjyjzj\sum_{j=1}^{s}x_jy_jz_j, whose flattening has rank ss, so the assertion also holds. We may therefore assume s,d≥2s,d\ge2.

Fix ε>0\varepsilon>0 and the constant in (5). For all sufficiently large integers kk, set

hk=⌊(skCε)1/(w+ε)⌋.h_k=\left\lfloor\left(\frac{s^k}{C_\varepsilon}\right)^{1/(w+\varepsilon)}\right\rfloor.

Then hk≥2h_k\ge2, R(⟨hk,hk,hk⟩)≤skR(\langle h_k,h_k,h_k\rangle)\le s^k, and

lim⁡k→∞log⁡hkk=log⁡sw+ε.\lim_{k\to\infty}\frac{\log h_k}{k}=\frac{\log s}{w+\varepsilon}.

References

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