a(n) = (3*n-2)!!! * Sum_{k=0..n} 1/(3*k-2)!!!.

A368790

a(n) = (3*n-2)!!! * Sum_{k=0..n} 1/(3*k-2)!!!.

Terms

    a(0) =1a(1) =2a(2) =9a(3) =64a(4) =641a(5) =8334a(6) =133345a(7) =2533556a(8) =55738233a(9) =1393455826a(10) =39016763129

External references