a(n) = T(n,n+1), where T is the array defined in A025564.

Terms

a(0) =1a(1) =4a(2) =13a(3) =40a(4) =120a(5) =356a(6) =1050a(7) =3088a(8) =9069a(9) =26620a(10) =78133a(11) =229384a(12) =673699a(13) =1979628a(14) =5820195a(15) =17121312a(16) =50394579a(17) =148413996a(18) =437324919a(19) =1289330520a(20) =3803175474a(21) =11223840012a(22) =33139076292a(23) =97889042384a(24) =289276841475a(25) =855205791076