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

Terms

a(0) =0a(1) =9a(2) =25a(3) =54a(4) =108a(5) =211a(6) =411a(7) =804a(8) =1582a(9) =3129a(10) =6213a(11) =12370a(12) =24672a(13) =49263a(14) =98431a(15) =196752a(16) =393378a(17) =786613a(18) =1573065a(19) =3145950a(20) =6291700a(21) =12583179a(22) =25166115a(23) =50331964a(24) =100663638a(25) =201326961a(26) =402653581a(27) =805306794a(28) =1610613192a(29) =3221225959