Number of words of length 4 in the n(n-1)/2 transpositions of S[ n ] equivalent to the identity.

Terms

a(0) =0a(1) =1a(2) =27a(3) =120a(4) =340a(5) =765a(6) =1491a(7) =2632a(8) =4320a(9) =6705a(10) =9955a(11) =14256a(12) =19812a(13) =26845a(14) =35595a(15) =46320a(16) =59296a(17) =74817a(18) =93195a(19) =114760a(20) =139860a(21) =168861a(22) =202147a(23) =240120a(24) =283200a(25) =331825a(26) =386451a(27) =447552a(28) =515620a(29) =591165