Triangle T(n,k) = number of permutations of n elements with k 2-cycles.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =3a(5) =3a(6) =15a(7) =6a(8) =3a(9) =75a(10) =30a(11) =15a(12) =435a(13) =225a(14) =45a(15) =15a(16) =3045a(17) =1575a(18) =315a(19) =105a(20) =24465a(21) =12180a(22) =3150a(23) =420a(24) =105a(25) =220185a(26) =109620a(27) =28350a(28) =3780a(29) =945