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

Terms

a(0) =4a(1) =4a(2) =5a(3) =9a(4) =21a(5) =53a(6) =133a(7) =325a(8) =773a(9) =1797a(10) =4101a(11) =9221a(12) =20485a(13) =45061a(14) =98309a(15) =212997a(16) =458757a(17) =983045a(18) =2097157a(19) =4456453a(20) =9437189a(21) =19922949a(22) =41943045a(23) =88080389a(24) =184549381a(25) =385875973a(26) =805306373a(27) =1677721605a(28) =3489660933a(29) =7247757317