Schur polynomials twisted by roots of unity and reciprocal pairs: torsion filters, fusion quotients, and total unimodularity at odd order

Abstract

Write ${\mu }_t$ for the $t$-th roots of unity and $z^{\pm 1}$ for $r$ free reciprocal pairs. We study ${\Phi }_{t,r}(\beta )=s_{\lambda }({\mu }_t,z^{\pm 1})$, $\beta =\lambda +\delta$, and the question the companion paper left open after $r=1$: when does it vanish? We factor the evaluation into classical branching followed by a torsion filter, and the shape depends on the parity of $t$: the point lies in the orthogonal group with determinant $(-1{)}^{t+1}$. For odd $t$ it sits in the identity component: an ordinary restriction $S{O}_{2R^{\prime }+1}\downarrow S{O}_{2m^{\prime }+1}\times S{O}_{2r}$, the filter an odd orthogonal character at a principal element of order $h+1$. For even $t$ in the other: a twining, a virtual expansion, and a torsion element regular but not principal; there we prove the filter, with its sign. One description covers both: the filter is nonzero exactly when the shifted point is regular semisimple. Both are minimal-level fusion projections: the even of type $C$, the odd's tensor sector of type $B$. Affine folding accounts for $0,\pm 1$; what it does not survives as conjectures. The highest surviving weight is the dominant vertex of the numerator's Newton polytope minus the denominator's --- the latter proved, the former conditional on a single-orbit property --- and the class there is conjecturally primitive, $\pm$ the generator of the rank-one quotient. For odd $t$ and one $\Lambda$ that numerator is a signed transversal count in $\{0,\pm 1\}$ by the equal-rank character formula, leaving one division. We invert it in closed form, along an arithmetic progression; the quotient is $\pm {\epsilon }_t\det M$ for an explicit $0/\pm 1$ matrix --- an interval matrix up to signs, hence totally unimodular, which settles (L1). The fibre count is a permanent, odd only when $1$, so at a dominant index a multi-hit fibre sums to zero. Two extremal statements remain. What is unproved is measured, in both parities.

80 pages, 13 figures; ancillary scripts and their archived output included