Number of involutions avoiding the pattern 21 (with a dot over the 1).

Terms

a(0) =1a(1) =0a(2) =1a(3) =1a(4) =4a(5) =9a(6) =31a(7) =94a(8) =337a(9) =1185a(10) =4540a(11) =17581a(12) =71875a(13) =299646a(14) =1299637a(15) =5760973a(16) =26357764a(17) =123241185a(18) =591877543a(19) =2902472734a(20) =14571525145a(21) =74613410169a(22) =390197960716