a(n) = 6*2^n - 4*n - 6.

Terms

a(0) =0a(1) =2a(2) =10a(3) =30a(4) =74a(5) =166a(6) =354a(7) =734a(8) =1498a(9) =3030a(10) =6098a(11) =12238a(12) =24522a(13) =49094a(14) =98242a(15) =196542a(16) =393146a(17) =786358a(18) =1572786a(19) =3145646a(20) =6291370a(21) =12582822a(22) =25165730a(23) =50331550a(24) =100663194a(25) =201326486a(26) =402653074a(27) =805306254a(28) =1610612618a(29) =3221225350