Number of permutations of length n within distance 3 of a fixed permutation.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =24a(5) =78a(6) =230a(7) =675a(8) =2069a(9) =6404a(10) =19708a(11) =60216a(12) =183988a(13) =563172a(14) =1725349a(15) =5284109a(16) =16177694a(17) =49526506a(18) =151635752a(19) =464286962a(20) =1421566698a(21) =4352505527a(22) =13326304313a(23) =40802053896a(24) =124926806216a(25) =382497958000