a(n) = 2^n - C(n,0) - C(n,1) - C(n,2) - C(n,3).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =6a(6) =22a(7) =64a(8) =163a(9) =382a(10) =848a(11) =1816a(12) =3797a(13) =7814a(14) =15914a(15) =32192a(16) =64839a(17) =130238a(18) =261156a(19) =523128a(20) =1047225a(21) =2095590a(22) =4192510a(23) =8386560a(24) =16774891a(25) =33551806a(26) =67105912a(27) =134214424a(28) =268431773a(29) =536866822