Number of permutations of length n within distance 2 of a fixed permutation.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =14a(5) =31a(6) =73a(7) =172a(8) =400a(9) =932a(10) =2177a(11) =5081a(12) =11854a(13) =27662a(14) =64554a(15) =150639a(16) =351521a(17) =820296a(18) =1914208a(19) =4466904a(20) =10423761a(21) =24324417a(22) =56762346a(23) =132458006a(24) =309097942a(25) =721296815a(26) =1683185225a(27) =3927803988a(28) =9165743600a(29) =21388759708