a(n) = 13*2^(n+1) - 19.

Terms

a(0) =7a(1) =33a(2) =85a(3) =189a(4) =397a(5) =813a(6) =1645a(7) =3309a(8) =6637a(9) =13293a(10) =26605a(11) =53229a(12) =106477a(13) =212973a(14) =425965a(15) =851949a(16) =1703917a(17) =3407853a(18) =6815725a(19) =13631469a(20) =27262957a(21) =54525933a(22) =109051885a(23) =218103789a(24) =436207597a(25) =872415213a(26) =1744830445a(27) =3489660909a(28) =6979321837a(29) =13958643693