a(n) = T(n,n+2), array T as in A050143.

Terms

a(0) =0a(1) =1a(2) =6a(3) =33a(4) =180a(5) =985a(6) =5418a(7) =29953a(8) =166344a(9) =927441a(10) =5188590a(11) =29113953a(12) =163786428a(13) =923511849a(14) =5217709266a(15) =29532022785a(16) =167417253648a(17) =950453221153