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

Terms

a(0) =-1a(1) =3a(2) =15a(3) =47a(4) =127a(5) =319a(6) =767a(7) =1791a(8) =4095a(9) =9215a(10) =20479a(11) =45055a(12) =98303a(13) =212991a(14) =458751a(15) =983039a(16) =2097151a(17) =4456447a(18) =9437183a(19) =19922943a(20) =41943039a(21) =88080383a(22) =184549375a(23) =385875967a(24) =805306367a(25) =1677721599a(26) =3489660927a(27) =7247757311a(28) =15032385535