Realizability conjecture for orthopositroids

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For a positroid M\mathscr{M} of type (k,n)(k,n), and for subsets I,J⊆[n]I,J\subseteq [n] of size k−1k-1, define

AIJ±(M)={ℓ∈[n]:Iℓ,Jℓ∈M and (−1)ℓ−1ϵIℓϵJℓ=±1}.A_{IJ}^{\pm}(\mathscr{M})=\big\{\ell\in[n]:I\ell,J\ell\in\mathscr{M}\text{ and }(-1)^{\ell-1}\epsilon_{I\ell}\epsilon_{J\ell}=\pm1\big\}.

The positroid M\mathscr{M} is an orthopositroid if, for every I,J∈([n]k−1)I,J\in\binom{[n]}{k-1},

AIJ+(M)=∅⟺AIJ−(M)=∅.A_{IJ}^{+}(\mathscr{M})=\emptyset\quad\Longleftrightarrow\quad A_{IJ}^{-}(\mathscr{M})=\emptyset.

Realizability conjecture for orthopositroids. Every orthopositroid M\mathscr{M} of type (k,n)(k,n) is realizable: there exists X∈OGr⁡+(k,n)X\in\operatorname{OGr}_{+}(k,n) such that M=MX\mathscr{M}=\mathscr{M}_X.

The motivation is that the positroid associated with any point of OGr⁡+(k,n)\operatorname{OGr}_{+}(k,n) is necessarily an orthopositroid, because the Plücker coordinates satisfy the orthogonal relations. The paper states this realizability claim while noting that the general discussion of realizability is left for future work; no resolution is provided here.

References

Primary source

Yassine El Maazouz and Yelena Mandelshtam, “The positive orthogonal Grassmannian”, arXiv:2412.14091 (2025).

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