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 A007972 Number of permutations that are 2 "block reversals" away from 12...n. 5
 2, 15, 52, 129, 266, 487, 820, 1297, 1954, 2831, 3972, 5425, 7242, 9479, 12196, 15457, 19330, 23887, 29204, 35361, 42442, 50535, 59732, 70129, 81826, 94927, 109540, 125777, 143754, 163591, 185412, 209345, 235522, 264079, 295156, 328897, 365450, 404967, 447604 (list; graph; refs; listen; history; text; internal format)
 OFFSET 3,1 LINKS Sean A. Irvine, Table of n, a(n) for n = 3..100 FORMULA a(n) = (n^4+6*n^3+11*n^2-12*n+6)/6 (conjectured). - Giovanni Resta, Jun 08 2015 Conjectured g.f.: (-2-5x+3x^2+x^3-x^4)/(-1+x)^5. - Benedict W. J. Irwin, Feb 20 2016 MATHEMATICA a[n_] := Block[{s, allb, r = Flatten[Table[{i, j}, {i, n}, {j, i + 1, n}], 1]}, allb[pp_] := Union@ Table[ s=pp; s[[Range @@ e]] = Reverse[ s[[ Range @@ e]]]; s, {e, r}]; Length[Flatten[allb /@ allb[Range[n]], 1] // Union] - 1]; a /@ Range[3, 15] (* Giovanni Resta, Jun 08 2015 *) CROSSREFS Cf. A007973, A007974, A007975. Column k=2 of A300003. Sequence in context: A248538 A248539 A248540 * A248541 A248542 A248543 Adjacent sequences:  A007969 A007970 A007971 * A007973 A007974 A007975 KEYWORD nonn AUTHOR EXTENSIONS a(9)-a(41) from Giovanni Resta, Jun 08 2015 STATUS approved

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Last modified August 16 19:58 EDT 2018. Contains 313809 sequences. (Running on oeis4.)