Number of fixed points in all 231-avoiding involutions in S_n.

Terms

a(0) =1a(1) =2a(2) =6a(3) =14a(4) =34a(5) =78a(6) =178a(7) =398a(8) =882a(9) =1934a(10) =4210a(11) =9102a(12) =19570a(13) =41870a(14) =89202a(15) =189326a(16) =400498a(17) =844686a(18) =1776754a(19) =3728270a(20) =7806066a(21) =16311182a(22) =34020466a(23) =70837134a(24) =147266674a(25) =305718158a(26) =633805938a(27) =1312351118a(28) =2714180722a(29) =5607318414