a(n) = ((n+1)*3*2^(n+1) + 29*2^n + (-1)^n)/9.

Terms

a(0) =4a(1) =9a(2) =21a(3) =47a(4) =105a(5) =231a(6) =505a(7) =1095a(8) =2361a(9) =5063a(10) =10809a(11) =22983a(12) =48697a(13) =102855a(14) =216633a(15) =455111a(16) =953913a(17) =1995207a(18) =4165177a(19) =8679879a(20) =18058809a(21) =37515719a(22) =77827641a(23) =161247687a(24) =333680185a(25) =689729991a(26) =1424199225a(27) =2937876935a(28) =6054710841a(29) =12467335623