6*2^n - n^2 - 5*n - 6.

Terms

a(0) =0a(1) =0a(2) =4a(3) =18a(4) =54a(5) =136a(6) =312a(7) =678a(8) =1426a(9) =2940a(10) =5988a(11) =12106a(12) =24366a(13) =48912a(14) =98032a(15) =196302a(16) =392874a(17) =786052a(18) =1572444a(19) =3145266a(20) =6290950a(21) =12582360a(22) =25165224a(23) =50330998a(24) =100662594a(25) =201325836a(26) =402652372a(27) =805305498a(28) =1610611806a(29) =3221224480