A product triangle sequence based on recursion:a=4; f(n,a)=(2*a+1)*f(n-1,a)+f(n-2,a).

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =9a(5) =1a(6) =1a(7) =80a(8) =80a(9) =1a(10) =1a(11) =711a(12) =6320a(13) =711a(14) =1a(15) =1a(16) =6319a(17) =499201a(18) =499201a(19) =6319a(20) =1a(21) =1a(22) =56160a(23) =39430560a(24) =350439102a(25) =39430560a(26) =56160a(27) =1a(28) =1a(29) =499121