Reversion of sequence of involutions (A000085).

Terms

a(0) =1a(1) =-2a(2) =4a(3) =-10a(4) =30a(5) =-104a(6) =392a(7) =-1568a(8) =6520a(9) =-27976a(10) =122944a(11) =-551680a(12) =2518912a(13) =-11684000a(14) =54957216a(15) =-261897024a(16) =1263216192a(17) =-6164172608a(18) =30416619200a(19) =-151750104800a(20) =765364073120