a(n) = (5*3^n-2*n-1)/4.

Terms

a(0) =1a(1) =3a(2) =10a(3) =32a(4) =99a(5) =301a(6) =908a(7) =2730a(8) =8197a(9) =24599a(10) =73806a(11) =221428a(12) =664295a(13) =1992897a(14) =5978704a(15) =17936126a(16) =53808393a(17) =161425195a(18) =484275602a(19) =1452826824a(20) =4358480491a(21) =13075441493a(22) =39226324500a(23) =117678973522a(24) =353036920589