a(n) = A027052(n, 2n-7).

Terms

a(0) =0a(1) =2a(2) =6a(3) =20a(4) =68a(5) =228a(6) =754a(7) =2456a(8) =7884a(9) =24982a(10) =78282a(11) =242998a(12) =748364a(13) =2289584a(14) =6966346a(15) =21098366a(16) =63651808a(17) =191406976a(18) =573998990a(19) =1717334182a(20) =5127933348a(21) =15286303526a(22) =45503354154a(23) =135287179508a(24) =401809091392