Second convolution of A111989.

Terms

a(0) =1a(1) =18a(2) =216a(3) =2136a(4) =18864a(5) =154656a(6) =1203072a(7) =9000576a(8) =65339136a(9) =463112704a(10) =3219204096a(11) =22019254272a(12) =148577427456a(13) =990973034496