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A086853
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Number of permutations of length n with exactly 2 rising or falling successions.
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8
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0, 0, 0, 2, 10, 48, 256, 1670, 12846, 112820, 1108612, 12032154, 142852450, 1840969784, 25587270600, 381460235918, 6071318154166, 102742200205980, 1841978156709676, 34874169034136930, 695294184953602602, 14560120360421802464, 319510983674891800240
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OFFSET
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0,4
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COMMENTS
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Permutations of 12...n such that exactly 2 of the following occur: 12, 23, ..., (n-1)n, 21, 32, ..., n(n-1).
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REFERENCES
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F. N. David, M. G. Kendall and D. E. Barton, Symmetric Function and Allied Tables, Cambridge, 1966, p. 263.
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LINKS
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FORMULA
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Coefficient of t^2 in S[n](t) defined in A002464.
Conjecture: (-514*n+2465)*a(n) +2*(257*n^2-955*n-1085)*a(n-1) +(-555*n^2+2483*n-1670)*a(n-2) +16*(-17*n^2+73*n-75)*a(n-3) +(354*n^2+528*n-2299)*a(n-4) +2*(-121*n^2+1045*n-1401)*a(n-5) +3*(67*n-115)*(n-4)*a(n-6)=0. - R. J. Mathar, Jun 06 2013
shorter recurrence: (n-3)*(n-2)*(n-4)^3*a(n) = (n-3)*(n^4-9*n^3+23*n^2-4*n-29)*(n-4)*a(n-1) - (n-1)*(n^4-12*n^3+57*n^2-125*n+104)*(n-4)*a(n-2) - (n-2)*(n-1)*(n^4-15*n^3+83*n^2-198*n+169)*a(n-3) + (n-3)^3*(n-2)^2*(n-1)*a(n-4). - Vaclav Kotesovec, Aug 10 2013
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MAPLE
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S:= proc(n) option remember; `if`(n<4, [1, 1, 2*t, 4*t+2*t^2]
[n+1], expand((n+1-t)*S(n-1) -(1-t)*(n-2+3*t)*S(n-2)
-(1-t)^2*(n-5+t)*S(n-3) +(1-t)^3*(n-3)*S(n-4)))
end:
a:= n-> ceil(coeff(S(n), t, 2)):
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MATHEMATICA
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s[n_] := s[n] = If[n<4, {1, 1, 2*t, 4*t+2*t^2}[[n+1]], Expand[(n+1-t)*s[n-1] - (1-t)*(n-2+3*t)*s[n-2] - (1-t)^2*(n-5+t)*s[n-3] + (1-t)^3*(n-3)*s[n-4]]]; t[n_, k_] := Ceiling[Coefficient[s[n], t, k]]; a[n_] := t[n, 2]; Table[a[n], {n, 0, 22}] (* Jean-François Alcover, Mar 11 2014, after Alois P. Heinz *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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