a(n) = A027052(n, 2n-1).

Terms

a(0) =0a(1) =2a(2) =4a(3) =8a(4) =18a(5) =42a(6) =102a(7) =256a(8) =658a(9) =1722a(10) =4570a(11) =12264a(12) =33212a(13) =90626a(14) =248892a(15) =687360a(16) =1907506a(17) =5316266a(18) =14873082a(19) =41751944a(20) =117567784a(21) =331979650a(22) =939807344a(23) =2666718976a(24) =7583071868a(25) =21605822594a(26) =61672362872a(27) =176338826728a(28) =505001067346