a(1) = 1; thereafter a(n) = 6*(2^(n-1) - 1).

Terms

a(0) =1a(1) =6a(2) =18a(3) =42a(4) =90a(5) =186a(6) =378a(7) =762a(8) =1530a(9) =3066a(10) =6138a(11) =12282a(12) =24570a(13) =49146a(14) =98298a(15) =196602a(16) =393210a(17) =786426a(18) =1572858a(19) =3145722a(20) =6291450a(21) =12582906a(22) =25165818a(23) =50331642a(24) =100663290a(25) =201326586a(26) =402653178a(27) =805306362a(28) =1610612730a(29) =3221225466