Differential Recursions, Projective Discriminants, and Algebraic Generating Functions

Abstract

We study two hierarchies of bivariate generating functions defined by differential recursions: a self-dual hierarchy and a reduced-symmetry hierarchy. Their rational coefficient functions produce palindromic and anti-palindromic triangles, including OEIS A336110 and A140136. For general differential order $p$, the reduced hierarchy decomposes into $p-1$ Horn-type companions. An explicit differential intertwiner of order $p-3$ relates the last companion to the normalized self-dual generating function. The companions share a principal symbol and a rationally parametrized characteristic curve with a projective $S_3$ action. At $p=2$ this construction yields the Narayana quadratic. At $p=3$ the symmetry lifts to $S_4$: two algebraic sectors are edge resolvents of one quartic root configuration, and the nonsymmetric Gross--Witten--Wadia generating function has degree four. For $p=4$ and $p=5$ we derive algebraic equations of degrees six and eight, respectively, and find Galois groups $S_6$ and $S_8$ at generic specializations. At $p=6$ we verify a degree-ten specialization with Galois group $S_{10}$. Their discriminants exhibit a universal cubic characteristic factor and a squared lower-order factor. A boundary factorization explains the observed algebraic degrees and discriminant monomials. We formulate conjectures on algebraicity and Galois groups in the reduced hierarchy and identify a possible mechanism for transcendence in the higher self-dual hierarchy.