a(n) = 2^(n-4)*(n+2)*(n+3)*(n+4)*(n+5)*(n+6)/15.

Terms

a(0) =3a(1) =21a(2) =112a(3) =504a(4) =2016a(5) =7392a(6) =25344a(7) =82368a(8) =256256a(9) =768768a(10) =2236416a(11) =6336512a(12) =17547264a(13) =47628288a(14) =127008768a(15) =333398016a(16) =862912512a(17) =2205220864a(18) =5571084288a(19) =13927710720a(20) =34487664640a(21) =84651540480a(22) =206108098560a(23) =498094571520