Number of degree-n permutations of order exactly 2.

Terms

a(0) =0a(1) =1a(2) =3a(3) =9a(4) =25a(5) =75a(6) =231a(7) =763a(8) =2619a(9) =9495a(10) =35695a(11) =140151a(12) =568503a(13) =2390479a(14) =10349535a(15) =46206735a(16) =211799311a(17) =997313823a(18) =4809701439a(19) =23758664095a(20) =119952692895a(21) =618884638911