Squares of numbers of trees.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =4a(5) =9a(6) =36a(7) =121a(8) =529a(9) =2209a(10) =11236a(11) =55225a(12) =303601a(13) =1692601a(14) =9979281a(15) =59923081a(16) =373262400a(17) =2364779641a(18) =15343033689a(19) =101095382025a(20) =677435994225