On $t$-extensions of the Hankel determinants of certain automatic sequences

TL;DR

This paper investigates how introducing a parameter t into Hankel determinants of certain automatic sequences affects their algebraic properties, revealing polynomial structures and degree bounds.

Contribution

It establishes that t-extensions of Hankel determinants for the period-doubling and paperfolding sequences are polynomials, with specific degree and coefficient properties, using combinatorial methods.

Findings

t-extensions of period-doubling sequence determinants are polynomials with a unique odd leading coefficient

All t-extensions of the paperfolding sequence determinants are degree ≤ 3 polynomials

The combinatorial set-up effectively analyzes the fixed points counted by t.

Abstract

In 1998, Allouche, Peyri\`ere, Wen and Wen considered the Thue--Morse sequence, and proved that all the Hankel determinants of the period-doubling sequence are odd integral numbers. We speak of $t$-extension when the entries along the diagonal in the Hankel determinant are all multiplied by~$t$. Then we prove that the $t$-extension of each Hankel determinant of the period-doubling sequence is a polynomial in $t$, whose leading coefficient is the {\it only one} to be an odd integral number. Our proof makes use of the combinatorial set-up developed by Bugeaud and Han, which appears to be very suitable for this study, as the parameter $t$ counts the number of fixed points of a permutation. Finally, we prove that all the $t$-extensions of the Hankel determinants of the regular paperfolding sequence are polynomials in $t$ of degree less than or equal to $3$.