a(n) = 28*2^n - 22.

Terms

a(0) =6a(1) =34a(2) =90a(3) =202a(4) =426a(5) =874a(6) =1770a(7) =3562a(8) =7146a(9) =14314a(10) =28650a(11) =57322a(12) =114666a(13) =229354a(14) =458730a(15) =917482a(16) =1834986a(17) =3669994a(18) =7340010a(19) =14680042a(20) =29360106a(21) =58720234a(22) =117440490a(23) =234881002a(24) =469762026a(25) =939524074a(26) =1879048170a(27) =3758096362a(28) =7516192746a(29) =15032385514