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A007972
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Number of permutations that are 2 "block reversals" away from 12...n.
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5
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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
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OFFSET
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3,1
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LINKS
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FORMULA
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a(n) = (n^4+6*n^3+11*n^2-12*n+6)/6 (conjectured). - Giovanni Resta, Jun 08 2015
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MATHEMATICA
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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 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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