a(n) = 15*2^(n+1) - (5*n^2+22*n+30).

Terms

a(0) =0a(1) =3a(2) =26a(3) =99a(4) =282a(5) =695a(6) =1578a(7) =3411a(8) =7154a(9) =14727a(10) =29970a(11) =60563a(12) =121866a(13) =244599a(14) =490202a(15) =981555a(16) =1964418a(17) =3930311a(18) =7862274a(19) =15726387a(20) =31454810a(21) =62911863a(22) =125826186a(23) =251655059a(24) =503313042a(25) =1006629255a(26) =2013261938a(27) =4026527571a(28) =8053059114a(29) =16106122487