a(n) = 6*4^n - 12*3^n + 7*2^n - 1.

Terms

a(0) =0a(1) =1a(2) =15a(3) =115a(4) =675a(5) =3451a(6) =16275a(7) =72955a(8) =316275a(9) =1340251a(10) =5590035a(11) =23054395a(12) =94314675a(13) =383578651a(14) =1553331795a(15) =6270493435a(16) =25253701875a(17) =101530450651a(18) =407669649555