0,6

Conjecture: a(n) <= A324918(n) for all n >= 13.

Table of n, a(n) for n=0..75.

C. Defant, Counting 3-stack-sortable permutations, arXiv:1903.09138 [math.CO], 2019.

(PARI) f(n) = binomial(3*n, n)*2/((n+1)*(2*n+1)) % 2; \\ A324964

a(n) = sum(k=0, n, f(k)); \\ Michel Marcus, Apr 02 2019

Cf. A324964, A324965, A324918.

Sequence in context: A074280 A000523 A124156 * A072749 A066490 A156685

Adjacent sequences: A324962 A324963 A324964 * A324966 A324967 A324968

nonn

Colin Defant, Mar 21 2019

approved