Explicit Hankel transform conjecture for the cubic Catalan family

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Let c(x)c(x) denote the Catalan generating function and define a sequence by

Fα(x)=1−x−x2−αx31−x−x2−x3c(x3(1−x−x2−αx3)(1−x−x2−x3)2).F_\alpha(x)=\frac{1-x-x^2-\alpha x^3}{1-x-x^2-x^3}c\left(\frac{x^3(1-x-x^2-\alpha x^3)}{(1-x-x^2-x^3)^2}\right).

Define

An(α)=[xn]1+(α−2)x2−(α−2)x3+(4α−5)x5−(α−1)(α−2)x81+3x3−(α−2)x6,A_n(\alpha)=[x^n]\frac{1+(\alpha-2)x^2-(\alpha-2)x^3+(4\alpha-5)x^5-(\alpha-1)(\alpha-2)x^8}{1+3x^3-(\alpha-2)x^6}, Bn=[xn]x(1−x+2x2−2x3+3x4−3x5+x6)(1−x)2(1−x3).B_n=[x^n]\frac{x(1-x+2x^2-2x^3+3x^4-3x^5+x^6)}{(1-x)^2(1-x^3)}.

Cubic-family Hankel-transform conjecture. The Hankel transform of the sequence with generating function Fα(x)F_\alpha(x) is hn=An(α)(2−α)Bnh_n=A_n(\alpha)(2-\alpha)^{B_n}. The formula is given as a further conjectural pattern after the quadratic-family examples.

References

Primary source

Paul Barry, “Generalized Catalan recurrences, Riordan arrays, elliptic curves, and orthogonal polynomials”, arXiv:1910.00875 (2019).

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