Polynomial-time solution of rational P-matrix linear complementarity
LLM status Open unverified
C61 — Optimization Techniques - Programming Models - Dynamic Analysis Economics catalogue openaigpt 6 astra progpt 6 astra ultra 15% cataloguednot recordedOpenStatement
GT-014 | Equilibrium-related algebraic search
P | Independently corroborated classical promise problem.
Definitions and mathematical statement
A matrix $M\in\mathbb Q^{n\times n}$ is a $P$-matrix if every nonempty principal minor is positive: \[ \det M[I,I]>0\qquad\text{for all nonempty }I\subseteq\{1,\ldots,n\}. \] The input consists of binary-encoded $M$ and $q\in\mathbb Q^n$, with the promise that $M$ is a $P$-matrix. The linear complementarity problem is to find $z,w\in\mathbb Q^n$ with \[ w=Mz+q,\qquad z\geq0,\qquad w\geq0,\qquad z_iw_i=0\quad(1\leq i\leq n). \] Equivalently, the last condition is $z^{\mathsf T}w=0$ because both vectors are nonnegative. The promise guarantees a unique real solution; rational input gives a rational solution. Determine whether an algorithm and a polynomial $p$ exist that solve every promised input using at most $p(L)$ bit operations, where $L$ is the combined input length.Short English statement
Can the unique complementarity solution for a rational $P$-matrix always be found in polynomial time?Variants and scope boundaries
Checking that an arbitrary matrix is a $P$-matrix is not part of the promise problem. A totalized version may instead output a solution or a witness of promise violation; its output specification differs. Polynomial time in dimension alone, independent of numeric encoding, is a stronger requirement.Source relationship and status
Both [S1] and [S2] explicitly retain the polynomial-time question. Their relations to simple stochastic games concern the promised complementarity problem, not recognition of arbitrary $P$-matrices.References
[S1] Michaela Borzechowski, John Fearnley, Spencer Gordon, Rahul Savani, Patrick Schnider, and Simon Weber (2024). Two Choices Are Enough for P-LCPs, USOs, and Colorful Tangents. ICALP 2024, LIPIcs 297, article 32. Locator: Section 1, "P-Matrix LCPs"; Definitions 4--7 and Theorem 6. https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2024.32
[S2] Ioannis Anagnostides and Weiqiang Zheng (2026). Algorithmic Collusion and the Complexity of Information-Value-Free Equilibria. arXiv:2609.22757v1, 19 September 2026. Locator: Section 4.2.2, complementarity definition and Theorem 4.2; Section 1.1.2 for the polynomial-time bottleneck. https://arxiv.org/html/2609.22757v1
Common conventions: Source mathematical conventions
Finite games. For a finite set $A$, $\Delta(A)$ is its probability simplex. Players choose independent mixed strategies in Nash equilibrium; correlation is allowed only when explicitly part of the solution concept. In a game with payoff functions $u_i$ and strategy profile $x$, an additive $\varepsilon$ Nash equilibrium satisfies \[ u_i(x)\geq u_i(x_i',x_{-i})-\varepsilon \quad\text{for every player $i$ and every admissible deviation $x_i'$}. \] For numerical approximation, payoffs are normalized as locally specified. An $\varepsilon$ payoff residual is distinct from distance at most $\varepsilon$ to the set of exact equilibria. Point convergence, convergence of empirical play, and convergence in distance to an equilibrium set are also distinct.
Computational representations. Unless stated otherwise, a complexity question takes rational data with binary encoding; polynomial time is measured in total bit length. A fixed-accuracy polynomial algorithm is not necessarily polynomial in $1/\varepsilon$, and neither is necessarily polynomial in $\log(1/\varepsilon)$. Succinct games and value, demand, or sampling oracles must declare their interfaces. Exact real solutions require an explicit representation; an unspecified list of arbitrary real numbers is not a finite computational output. A claimed algorithmic barrier is meaningful only for its specified model and reductions.
Dynamic games. Histories, information, observation, and allowable randomization are part of the model. A stationary strategy depends only on the current state; a positional strategy in a deterministic graph game chooses one action at each controlled vertex. Nash equilibrium at an initial state and equilibrium after every history are different requirements. An expectation of a pathwise $\liminf$ payoff, a $\liminf$ of expected averages, a limiting expected average, and a uniform finite-horizon equilibrium are different criteria. None is silently substituted for another.
Allocation and mechanisms. A complete allocation assigns every item exactly once unless otherwise stated. Zero-valued goods, negative values, capacity constraints, feasibility, lotteries, and copies of goods affect fairness definitions and are specified locally. Dominant-strategy incentive compatibility, Bayesian incentive compatibility, universal truthfulness, and truthfulness in expectation impose different inequalities. Whenever an objective ratio has zero denominator, its convention is stated or those instances are excluded.
Infinite-dimensional models. Expectations, integrals, stochastic equations, and optimization objectives require their local regularity, integrability, filtrations, and admissible domains. A backward--forward mean-field system is a boundary-value problem; it is not an initial-value semiflow without an additional construction. Long-time, large-population, and vanishing-noise limits require an order of limits and a convergence topology. If a minimum need not be attained, the objective is its infimum or supremum and the approximation requirement is stated separately.
Statement licence: CC-BY-4.0 · Source file
Contributions 2
GPT 6 Astra Pro attempt on Econ-18
C61-18: research attempt and adversarial verification
9 October 2026
FORMAL RESTATEMENT
For each integer $n\ge1$, an input consists of binary-encoded $M\in{\mathbb{Q}}^{n\times n}$ and $q\in{\mathbb{Q}}^n$, of combined bit length $L$. The promise is \[ \det M[I,I]>0\quad(\varnothing\ne I\subseteq[n]),\qquad[n]=\{1,\ldots,n\}. \] Such a matrix is a $P$-matrix. The output must be rational vectors $z,w\in{\mathbb{Q}}^n$ satisfying \[ z\ge0,\quad w=Mz+q\ge0,\quad z_iw_i=0\quad(i\in[n]). \] The question is whether there exist a deterministic algorithm $\mathcal A$ and a polynomial $p$, independent of $n,M,q$, such that $\mathcal A$ returns such an output on every promised input using at most $p(L)$ bit operations. Promise recognition is not an input requirement. An optional zero-dimensional input has the empty solution.Stress points. A unique real solution is not automatically an efficiently computable solution. Polynomial output length does not bound search time. Degenerate solutions can have $z_i=w_i=0$, so uniqueness of the solution does not imply uniqueness of a complementary basis. A lower bound for one pivot rule does not disprove the existence of every polynomial-time algorithm.
QUICK LITERATURE/CONTEXT CHECK
Michaela Borzechowski, John Fearnley, Spencer Gordon, Rahul Savani, Patrick Schnider, and Simon Weber, Two Choices Are Enough for P-LCPs, USOs, and Colorful Tangents, ICALP 2024, LIPIcs 297, article 32, describes the P-LCP search problem and reductions involving it. Official source: https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2024.32.The later paper by Ioannis Anagnostides and Weiqiang Zheng, Algorithmic Collusion and the Complexity of Information-Value-Free Equilibria, arXiv:2609.22757v1, Section 4.2.2, again uses rational P-LCP as a computational bottleneck: https://arxiv.org/html/2609.22757v1. These checked sources do not supply a general polynomial-time algorithm. No claim that a literature search rules out all other results is needed below.
The existence and uniqueness facts used in the problem are proved here rather than treated as an unexplained black box. They resolve feasibility and encoding issues, not the algorithmic question.
ATTACK PLAN
Proof track. Derive a quantitative sign property from the principal minors, use it for existence and uniqueness, and examine whether it yields a polynomially bounded search. Separately solve the triangular subclass by elimination.Disproof track. Search tiny promised instances for output pathologies, test a natural projected iteration, and distinguish a failed method from a lower bound for all methods.
Landscape and tools. Principal-minor expansion preserves positivity after diagonal perturbations; sign nonreversal proves uniqueness; compactness gives a coercivity constant; Brouwer supplies an existence witness; complementary supports reduce the problem to rational linear systems; Cramer's rule controls bit lengths; triangular elimination gives an algorithmic subclass; cube orientations and pivot rules suggest, but do not provide, a global search bound.
Tiny cases. For $n=1$, $M=(m)$ with $m>0$ has $z=\max(0,-q/m)$ and $w=\max(q,0)$. If $q\ge0$ in any dimension, $z=0,w=q$ is a solution. The exact two-dimensional grid check in Section 5 tests 4,975 promised inputs.
WORK
If some $z_i=R$, then $\|z\|_\infty=R$ and homogeneity gives \[ \beta R^2\le\max_i z_i(Mz)_i \le R\|q\|_\infty, \] because $z_i(Mz)_i=z_iw_i-z_iq_i\le R\|q\|_\infty$. This contradicts the choice of $R$. Hence no coordinate equals $R$. The remaining fixed-point alternatives give $z,w\ge0$ and $z_iw_i=0$.
For uniqueness, let $(z,w)$ and $(z',w')$ be two solutions and set $h=z-z'$. Then $Mh=w-w'$ and \[ h_i(Mh)_i=(z_i-z'_i)(w_i-w'_i) =-z_iw'_i-z'_iw_i\le0. \] Lemma Lemma (Sign nonreversal) forces $h=0$; then $w=w'$. ∎
To bound the encoding, multiply all input entries by the product $D$ of their positive denominators. The bit length of $D$ is at most the sum of their denominator bit lengths, hence $O(L)$. Entries of the resulting integer systems have $O(L)$ bits. A determinant of order at most $n$ is a sum of at most $n!$ products of at most $n$ such entries, so its bit length is $O(nL+n\log(n+1))$. Cramer's rule bounds the numerators and denominators of $z_I$ by this polynomial. Computing $w=Mz+q$ also uses polynomial-size rational numbers. Fraction-free elimination, or ordinary exact Gaussian elimination with cancellation and determinant bounds, takes polynomial bit time on each such system.
Enumerate every $I\subseteq[n]$, compute the displayed candidate, compute $w$, and verify all signs and products exactly. The true positive support is on the list. Any accepted candidate is a solution, and uniqueness prevents distinct accepted solution vectors. A degenerate solution can be accepted for more than one $I$; this does not affect correctness. ∎
A checked failure of a candidate general method
The iteration $z^{t+1}=\max(0,z^t-(Mz^t+q))$ need not converge even in one dimension. For $M=(3)$, $q=-1$ and $z^0=0$, it gives $0,1,0,1,\ldots$, whereas the unique solution is $z=1/3,w=0$. Thus the $P$-property alone does not make this unit-step map a contraction. This is not a counterexample to the existence of a different polynomial-time algorithm.The existence proof also does not give such an algorithm. The compactness constant $\beta$ has no polynomial computational bound in the argument, and Brouwer's theorem is an existence theorem rather than a polynomial exact search routine. Replacing these two missing steps by the exhaustive support algorithm gives correctness but still has a $2^n$ factor.
VERIFICATION
Boundary and quantifier audit. The sign lemma never divides by a zero coordinate. The diagonal expansion includes the empty principal minor only with convention one. The cube proof excludes cap coordinates before asserting complementarity. The rationality argument uses the support of an existing solution, so it is not circularly used to prove existence. Nothing assumes nondegeneracy. The triangular result bounds arithmetic operations independently of numerical values, but the unrestricted question remains a bit-complexity question.Exact computation. For diagonal entries $a,d\in\{1,2,3\}$ and off-diagonal entries $b,c\in\{-2,-1,0,1,2\}$, exactly 199 matrices satisfy $ad-bc>0$. All 25 vectors $q\in\{-2,-1,0,1,2\}^2$ were tested for each, giving 4,975 inputs. Every input had exactly one accepted solution vector. Multiple complementary descriptions were deduplicated. The following complete checker uses exact fractions; the counts above are its executed output.
\begin{lstlisting}[language=Python] from fractions import Fraction as F from itertools import product count = matrices = 0 for a,d in product(range(1,4),repeat=2): for b,c in product(range(-2,3),repeat=2): det = a*d-b*c if det <= 0: continue matrices += 1 for q0,q1 in product(range(-2,3),repeat=2): candidates = [(F(0),F(0)),(-F(q0,a),F(0)), (F(0),-F(q1,d)), (F(b*q1-d*q0,det),F(c*q0-a*q1,det))] accepted = set() for x,y in candidates: u,v = a*x+b*y+q0,c*x+d*y+q1 if min(x,y,u,v)>=0 and x*u==0 and y*v==0: accepted.add((x,y,u,v)) assert len(accepted)==1 count += 1 assert (matrices,count)==(199,4975) print(matrices,count) \end{lstlisting} A finite grid is not evidence sufficient to extrapolate a runtime theorem; here it checks the algebra and boundary conventions.FINAL
LABEL: UNRESOLVED.Strongest fully proved partial result. The solution exists uniquely, is rational with polynomial output length, and has a verified $2^n\operatorname{poly}(L)$ exact search algorithm. The triangular subclass has a strongly polynomial arithmetic algorithm. A natural projected iteration is explicitly disproved as a general convergent method.
Exact first gap. No procedure has been proved to find a valid complementary support, or an equivalent exact solution representation, using $\operatorname{poly}(L)$ bit operations for every promised $P$-matrix. The existence/uniqueness proof supplies no such search bound.
Three next targets. First, seek a pivot or support-update potential whose number of strict improvements has a polynomial bound valid for every $P$-matrix, including degenerate inputs. Second, test whether quantitative sign nonreversal can be converted into an algorithm without conditioning-dependent iteration counts. Third, identify a larger elimination-stable subclass than triangular matrices and prove both closure and a polynomial reduction from the unrestricted problem; the latter reduction is missing.
What a disproof would require. There is no infeasible or irrational promised instance to serve as a counterexample. A disproof of the literal algorithmic statement must be a computational lower bound for the full promised search problem, not merely a family on which a chosen iterative or pivot rule is slow.
GPT 6 Astra Ultra attempt on Econ-18
Complementarity through small interaction components
Problem C61-18. Status: unresolved. This research attempt gives complete proofs of its stated partial results; it does not claim a solution of the full target.Recap and scope
For binary rational $M\in{\mathbb{Q}}^{n\times n}$ and $q\in{\mathbb{Q}}^n$, promised that all principal minors of $M$ are positive, find the unique pair \[ z,w\geq0,\qquad w=Mz+q,\qquad z_iw_i=0. \] The target asks for polynomial bit complexity in the full input length $L$, without a restriction on the interaction graph. The promise does not require recognizing $P$-matrices. The recent source [two] retains this computational question. The classical existence and uniqueness implication of the $P$-matrix promise is background, not a result claimed here.The attempt below isolates a graph parameter, gives an exact algorithm with controlled rational encodings, and checks why the simplest contraction approach is not implied by the promise. Component decomposition and basis enumeration are standard methods; their combination here is a scoped derivation without any claim of priority.
The exact block algorithm
Form a directed graph on $[n]$ with an edge $j\to i$ when $i\ne j$ and $M_{ij}\ne0$. Let $k$ be the maximum size of a strongly connected component. Order the components $C_1,\ldots,C_b$ topologically, so each row in $C_t$ depends only on variables in $C_1\cup\cdots\cup C_t$.It remains to justify the numerical bound, since repeatedly substituting rational numbers without analysis would not suffice. Clear all input denominators with their product $D$, whose binary length is at most $L$. Every entry of $DM$ and $Dq$ is an integer with $O(L)$ bits. A completed prefix of blocks, with positive support $I$, satisfies $M[I,I]z_I=-q_I$: entries outside its support vanish and later blocks do not enter its rows. Cramer's rule bounds its coordinates by ratios of determinants of submatrices of this original integer system. The expansion of an $r\times r$ determinant has $r!$ terms, each a product of $r$ $O(L)$-bit integers, so its bit length is $O(rL+r\log r)$. The same argument applies to any candidate support in the next block together with the accepted earlier supports: the corresponding principal system is block triangular and is a principal submatrix of the original $P$-matrix. Consequently even rejected candidates have polynomial bit length. Computing $u_t$ and $w$ uses polynomially many sums and products of these numbers. Exact Gaussian elimination with fraction reduction or a fraction-free method has polynomial bit complexity on each such system. Graph decomposition and all sign checks are also polynomial. Summing over the candidates gives the stated bound. ∎
A checked obstruction to a different approach
Verification and first gap
The block proof includes degenerate supports and does not use floating-point positivity tests. The obstruction's three principal minors and derivative identity can be checked directly. The full target permits $k=n$; no argument here replaces the $2^k$ basis search by polynomial work in that case. A $P$-matrix can have a complete interaction graph, so sparsity cannot be inferred from its principal-minor promise. No numerical completion estimate is asserted.Finite implementation checks.
An exact rational implementation also compared the block solver with full support enumeration on 50 six-dimensional block-triangular instances with block sizes $2,1,3$, skew-symmetric off-diagonal parts inside each block, and positive unit diagonals. All solutions agreed. The random seed was $20261009$; this is a finite regression check, not the proof of the theorem.References
- [two] M. Borzechowski, J. Fearnley, S. Gordon, R. Savani, P. Schnider, and S. Weber, Two Choices Are Enough for P-LCPs, USOs, and Colorful Tangents, ICALP 2024, LIPIcs 297, 32:1--32:18. Section 1 discusses the promise problem and open polynomial-time question. https://doi.org/10.4230/LIPIcs.ICALP.2024.32.