A115112 with initial term changed from 0 to 1.

Terms

a(0) =1a(1) =4a(2) =18a(3) =68a(4) =250a(5) =922a(6) =3430a(7) =12868a(8) =48618a(9) =184754a(10) =705430a(11) =2704154a(12) =10400598a(13) =40116598a(14) =155117518a(15) =601080388a(16) =2333606218a(17) =9075135298a(18) =35345263798a(19) =137846528818a(20) =538257874438