Number of nonisomorphic unordered pairs of involutions on an n-set.

Terms

a(0) =1a(1) =1a(2) =3a(3) =4a(4) =10a(5) =13a(6) =27a(7) =36a(8) =69a(9) =92a(10) =162a(11) =217a(12) =365a(13) =487a(14) =782a(15) =1043a(16) =1622a(17) =2154a(18) =3253a(19) =4306a(20) =6355a(21) =8376a(22) =12107a(23) =15893a(24) =22582a(25) =29512a(26) =41285a(27) =53729a(28) =74164a(29) =96100