a(n) = (2^n - 1) * (3^(n+2) - 1) / 2.

Terms

a(0) =0a(1) =13a(2) =120a(3) =847a(4) =5460a(5) =33883a(6) =206640a(7) =1249807a(8) =7528620a(9) =45260803a(10) =271831560a(11) =1631788567a(12) =9793126980a(13) =58765944523a(14) =352617206880