a(n) = Sum_{k=0..n} (-1)^k * binomial(2*n,k) * binomial(3*n,n-k).

Terms

a(0) =1a(1) =1a(2) =-3a(3) =-17a(4) =-19a(5) =126a(6) =591a(7) =344a(8) =-5907a(9) =-22373a(10) =2122a(11) =280842a(12) =854063a(13) =-810692a(14) =-13254552a(15) =-31693392a(16) =67250413a(17) =615932985a(18) =1101123015a(19) =-4368359919a(20) =-28043889894a(21) =-33371056204a(22) =254637122506a(24) =693586015791