Main diagonal of table A083087.

Terms

a(0) =1a(1) =5a(2) =25a(3) =90a(4) =247a(5) =766a(6) =2258a(7) =5860a(8) =16526a(9) =45639a(10) =124043a(11) =313327a(12) =837221a(13) =2216256a(14) =5545541a(15) =14524810a(16) =37810204a(17) =97907017a(18) =242993558a(19) =625252309a(20) =1602714963a(21) =3962518559a(22) =10109705767a(23) =25718726896a(24) =63402237401a(25) =160711911464