Number of permutations that are 3 "block reversals" away from 12...n.

Terms

a(0) =2a(1) =55a(2) =389a(3) =1563a(4) =4642a(5) =11407a(6) =24600a(7) =48204a(8) =87758a(9) =150707a(10) =246787a(11) =388445a(12) =591294a(13) =874603a(14) =1261822a(15) =1781142a(16) =2466090a(17) =3356159a(18) =4497473a(19) =5943487a(20) =7755722a(21) =10004535a(22) =12769924a(23) =16142368a(24) =20223702a(25) =25128027a(26) =30982655a(27) =37929089a(28) =46124038a(29) =55740467