a(n) equals the coefficient of x^n in Sum_{m>=0} (x^m + 1 + 1/x^m)^m for n >= 1.

Terms

a(0) =1a(1) =2a(2) =6a(3) =17a(4) =45a(5) =129a(6) =357a(7) =1026a(8) =2908a(9) =8380a(10) =24068a(11) =69670a(12) =201643a(13) =585956a(14) =1704525a(15) =4969937a(16) =14508939a(17) =42424376a(18) =124191258a(19) =363992450a(20) =1067892560a(21) =3136066153a(22) =9217554129a(23) =27114308763a(24) =79818194926a(25) =235128636132a(26) =693085100406