Measure-theoretic defect conjecture for Butson matrices

Let HCN(s)H\in C_N(s), with sNs\in\mathbb N minimal, and let μ\mu be the probability measure associated to the random variable

φ(a,b)=#{(i,j)aibjHij=1},\varphi(a,b)=\#\{(i,j)\mid a_i b_j H_{ij}=1\},

for uniformly distributed (a,b)ZsN×ZsN(a,b)\in\mathbb Z_s^N\times\mathbb Z_s^N. Write supp(μ)conv\overline{\operatorname{supp}(\mu)}^{\,\operatorname{conv}} for the closed convex hull of the support of μ\mu.

Measure-theoretic defect conjecture. For HCN(s)H\in C_N(s), with ss minimal,

d(H)supp(μ)conv.d(H)\in\overline{\operatorname{supp}(\mu)}^{\,\operatorname{conv}}.

This is presented as a reformulation of the Butson defect sandwich conjecture, expressing the defect through the distribution of unit-entry counts under row and column multiplication. The source gives no resolution.

Sources & referencesView supporting material

Primary source

Teodor Banica, “First order deformations of the Fourier matrix”, arXiv:1302.4153 (2013).

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