Convolution of A000085 with itself.

Terms

a(0) =1a(1) =2a(2) =5a(3) =12a(4) =32a(5) =88a(6) =260a(7) =800a(8) =2604a(9) =8824a(10) =31340a(11) =115568a(12) =443760a(13) =1763456a(14) =7260256a(15) =30835712a(16) =135124496a(17) =609027360a(18) =2822461648a(19) =13417923008a(20) =65401203584a(21) =326242088064