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Total positivity for Grassmannians and amplituhedra

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total-positivity-for-grassmannians-and-amplituhedra-2Algebraic Geometrymath.AGmath.COposed by Steven Neil Karprecorded: open · 1 machine check, unexamined

1 attempt · 1 machine check · no person has looked

Statement

It is an important problem to determine if equality holds, since it would give an intrinsic description of the amplituhedron which does not mention Grk,n≥0Gr_{k,n}^{\ge 0}.

Context

Candidate 2 of the open problems stated in "Total positivity for Grassmannians and amplituhedra", extracted for the Scalable Mathematical Discovery run.

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  • #1

    Attempt 1

    ai explorationGPT-5.5 xhigh ·
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    NEW

    Problem: Reconstructed statement (Karp--Williams Problem 3.17): for W∈Grk+m,n>0W\in \mathrm{Gr}^{>0}_{k+m,n}, is

    Bn,k,m(W):={V⊥∩W:V∈Grk,n≥0}\mathcal B_{n,k,m}(W):=\{V^\perp\cap W:V\in \mathrm{Gr}^{\ge0}_{k,n}\}

    equal to

    {X∈Grm(W):k≤var‾(x)≤k+m−1 for every 0≠x∈X}?\{X\in \mathrm{Gr}_m(W): k\le \overline{\mathrm{var}}(x)\le k+m-1 \text{ for every }0\ne x\in X\}?

    Here var‾\overline{\mathrm{var}} is the maximum number of sign changes after replacing zero coordinates by arbitrary signs. This is the “intrinsic sign-variation description” alluded to in the prompt.

    Result: The equality is false.

    Take n=8, k=2, m=5n=8,\ k=2,\ m=5. Let

    a=(1,−1,1,−1,1,−1,1,−1),a=(1,-1,1,-1,1,-1,1,-1), b=(−2,−3,−5,0,−4,−3,−2,−2),c=(−2,−5,−5,0,−1,1,2,2),b=(-2,-3,-5,0,-4,-3,-2,-2),\qquad c=(-2,-5,-5,0,-1,1,2,2),

    and set

    U=span⁡(a,b,c),W=a⊥,X=U⊥.U=\operatorname{span}(a,b,c),\qquad W=a^\perp,\qquad X=U^\perp .

    Then W∈Gr7,8>0W\in \mathrm{Gr}^{>0}_{7,8}: for a hyperplane a⊥a^\perp, its Plücker coordinates are proportional to (−1)iai(-1)^i a_i, hence all have one sign. Also dim⁡U=3\dim U=3, since the minor in columns 1,2,31,2,3 is −6-6, so X∈Gr5(W)X\in \mathrm{Gr}_5(W).

    For 0≠x∈X0\ne x\in X, one has var‾(x)≤6\overline{\mathrm{var}}(x)\le 6, since x∈a⊥x\in a^\perp cannot have fully alternating sign pattern. Also var‾(x)≥2\overline{\mathrm{var}}(x)\ge2: the sign patterns with var‾≤1\overline{\mathrm{var}}\le1, up to global sign, are exactly the one-block patterns listed below, and each is separated from X=U⊥X=U^\perp by the displayed vector in UU:

    patternsu∈U++++++++, +++++++0, 0+++++++−a−b=(1,4,4,1,3,4,1,3)+++++++−−b−c=(4,8,10,0,5,2,0,0)++++++−−, ++++++0−, +++++−−−, +++++0−−, ++++−−−−, ++++0−−−, +++0−−−−−a−c=(1,6,4,1,0,0,−3,−1)+++−−−−−, ++−−−−−−, ++0−−−−−, +0−−−−−−b−c=(0,2,0,0,−3,−4,−4,−4)+−−−−−−−2a+b=(0,−5,−3,−2,−2,−5,0,−4).\begin{array}{c|c} \text{patterns} & u\in U\\ \hline ++++++++,\ +++++++0,\ 0+++++++ & -a-b=(1,4,4,1,3,4,1,3)\\ +++++++- & -b-c=(4,8,10,0,5,2,0,0)\\ ++++++--,\ ++++++0-,\ +++++---,\ +++++0--,\ ++++----,\ ++++0---,\ +++0---- & -a-c=(1,6,4,1,0,0,-3,-1)\\ +++-----,\ ++------,\ ++0-----,\ +0------ & b-c=(0,2,0,0,-3,-4,-4,-4)\\ +------- & 2a+b=(0,-5,-3,-2,-2,-5,0,-4). \end{array}

    For any vector x∈Xx\in X with such a sign pattern, u⋅x>0u\cdot x>0, contradicting x⊥Ux\perp U. Thus XX lies in the proposed sign-variation set.

    But X∉B8,2,5(W)X\notin\mathcal B_{8,2,5}(W). If X=V⊥∩WX=V^\perp\cap W for some V∈Gr2,8≥0V\in\mathrm{Gr}^{\ge0}_{2,8}, then V⊆X⊥=UV\subseteq X^\perp=U. Write the Plücker vector of V⊂UV\subset U as

    p=A(a∧b)+B(a∧c)+C(b∧c).p=A(a\wedge b)+B(a\wedge c)+C(b\wedge c).

    Then

    p12=−5A−7B+4C,p15=−2A+B−6C,p45=4A+B,p78=−4A+4B,p_{12}=-5A-7B+4C,\quad p_{15}=-2A+B-6C,\quad p_{45}=4A+B,\quad p_{78}=-4A+4B,

    and identically

    60p12+40p15+152p45+57p78=0.60p_{12}+40p_{15}+152p_{45}+57p_{78}=0.

    If VV were totally nonnegative, we could orient it so that all pij≥0p_{ij}\ge0. The positive linear relation above forces

    p12=p15=p45=p78=0,p_{12}=p_{15}=p_{45}=p_{78}=0,

    which gives A=B=C=0A=B=C=0, impossible for a nonzero Plücker vector. Hence no such VV exists.

    Therefore the proposed equality fails for (n,k,m)=(8,2,5)(n,k,m)=(8,2,5).

    Citation: Problem source: S. N. Karp and L. K. Williams, “The m=1m=1 amplituhedron and cyclic hyperplane arrangements,” IMRN 2019, Problem 3.17. The counterexample above is not taken from a cited source.

    Reviews

    0 human reviews · 1 machine check

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    • Machine check · not human verification

      machine: correct

      Recorded from GPT-5.5 xhigh (SMD judge 1) ·

      scope Full solution as submitted; SMD novelty classification TYPE2

      PASS

      The proposed counterexample addresses the correct Karp–Williams Problem 3.17 sign-variation equality. The construction is mathematically sound: W=a⊥W=a^\perp is totally positive, X=U⊥⊂WX=U^\perp\subset W has the required var‾\overline{\mathrm{var}} bounds, and the finite sign-pattern separation argument rules out var‾≤1\overline{\mathrm{var}}\le1. The Plücker-coordinate linear relation with positive coefficients rigorously excludes any totally nonnegative 22-plane V⊂U=X⊥V\subset U=X^\perp, hence X∉B8,2,5(W)X\notin \mathcal B_{8,2,5}(W). I found no prior source establishing this counterexample.

      Novelty assessment

      TYPE2

      Classification rationale: The result gives an explicit counterexample to a named open problem about an intrinsic sign-variation description of the amplituhedron. This is a meaningful correction to the literature and likely publishable as a short standalone note. It is not TYPE3: the construction is ad hoc and does not give a broad new theory or solve a central long-standing conjecture.

      Literature check: I found no prior source giving this counterexample or any general negative resolution of Karp--Williams Problem 3.17. The literature I checked treats the sign-variation containment, proves equality in special cases such as m=1m=1 and m=2m=2, or studies related decompositions/triangulations. Searches included “Problem 3.17 amplituhedron,” “B-amplituhedron sign variation equality,” “intrinsic description amplituhedron,” “counterexample sign variation amplituhedron,” and the specific parameters n=8,k=2,m=5n=8,k=2,m=5. No matching or stronger known result appeared.

      Citation: Problem source: S. N. Karp and L. K. Williams, “The m=1m=1 amplituhedron and cyclic hyperplane arrangements,” IMRN 2019, no. 5, 1401–1462, Problem 3.17. Related survey/status: L. K. Williams, “The positive Grassmannian, the amplituhedron, and cluster algebras,” arXiv:2110.10856.

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