a(n) = 123*2^n - 135.

Terms

a(0) =111a(1) =357a(2) =849a(3) =1833a(4) =3801a(5) =7737a(6) =15609a(7) =31353a(8) =62841a(9) =125817a(10) =251769a(11) =503673a(12) =1007481a(13) =2015097a(14) =4030329a(15) =8060793a(16) =16121721a(17) =32243577a(18) =64487289a(19) =128974713a(20) =257949561a(21) =515899257a(22) =1031798649a(23) =2063597433a(24) =4127195001a(25) =8254390137a(26) =16508780409a(27) =33017560953a(28) =66035122041a(29) =132070244217