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A050407
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a(n) = n*(n^2 - 6*n + 11)/6.
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18
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0, 1, 1, 1, 2, 5, 11, 21, 36, 57, 85, 121, 166, 221, 287, 365, 456, 561, 681, 817, 970, 1141, 1331, 1541, 1772, 2025, 2301, 2601, 2926, 3277, 3655, 4061, 4496, 4961, 5457, 5985, 6546, 7141, 7771, 8437, 9140, 9881, 10661, 11481, 12342, 13245, 14191
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refs;
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OFFSET
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0,5
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COMMENTS
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Number of invertible shuffles of n-2 cards. - Adam C. McDougall (mcdougal(AT)stolaf.edu) and David Molnar (molnar(AT)stolaf.edu), Apr 09 2002
If Y is a 3-subset of an n-set X then, for n>=3, a(n-2) is the number of (n-3)-subsets of X which have neither one element nor two elements in common with Y. - Milan Janjic, Dec 28 2007
Let A be the Hessenberg n X n matrix defined by: A[1,j]=j mod 2, A[i,i]:=1, A[i,i-1]=-1, and A[i,j]=0 otherwise. Then, for n>=4, a(n+1)=-coeff(charpoly(A,x),x^(n-3)). - Milan Janjic, Jan 24 2010
Starting with offset 3: (1, 2, 5, 11, 21, ...) = triangle A144257 * [1,2,3,...]. - Gary W. Adamson, Feb 18 2010
(1 + 2x + 5x^2 + 11x^3 + ...) = (1 + 2x + 3x^2 + ...)*(1 + 2x^2 + 3x^3 + ...). - Gary W. Adamson, Jul 26 2010
Starting (1, 2, 5, 11, ...) = binomial transform of [1, 1, 2, 1, 0, 0, 0, ...]. - Gary W. Adamson, Aug 25 2010
For n > 0, a(n) is the number of valid hook configurations of permutations of [n-1] that avoid the patterns 132 and 321. - Colin Defant, Apr 28 2019
For n >= 1, a(n+2) is the number of Grassmannian permutations that avoid a pattern, sigma, where sigma is a pattern of size 4 with exactly one descent. - Jessica A. Tomasko, Nov 15 2022
The number of bigrassmannian permutations in S_n is a(n+2) = binomial(n+1, 3) + 1. - Joshua Swanson, Jan 08 2024
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LINKS
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Robert DiSario, Problem 10931, Amer. Math. Monthly, 109 (No. 3, 2002), 298.
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FORMULA
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Diagonal sums of square array A086460 (starting 1, 1, 2, ...).
a(n+2) = 1 + n*(n+1)*(n-1)/6 = Sum_{k=0..n} (0^k + (n-k)*k). (End)
a(n) = binomial(n-1,3) + binomial(n-1,0), n>=0. - Zerinvary Lajos, Jul 24 2006
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MAPLE
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MATHEMATICA
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LinearRecurrence[{4, -6, 4, -1}, {0, 1, 1, 1}, 50] (* Vincenzo Librandi, Jun 22 2012 *)
Join[{0, 1, 1}, Nest[Accumulate, Range[0, 50], 2]+1] (* Harvey P. Dale, Sep 23 2017 *)
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PROG
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(Magma) I:=[0, 1, 1, 1]; [n le 4 select I[n] else 4*Self(n-1)-6*Self(n-2)+4*Self(n-3)-Self(n-4): n in [1..50]]; // Vincenzo Librandi, Jun 22 2012
(Haskell)
a050407 n = n * (n ^ 2 - 6 * n + 11) `div` 6
(Python) for n in range(0, 50): print(n*(n**2 - 6*n + 11)/6, end=', ') # Stefano Spezia, Jan 05 2019
(Sage) [n*(n^2-6*n+11)/6 for n in (0..50)] # G. C. Greubel, Oct 30 2019
(GAP) List([0..50], n-> n*(n^2-6*n+11)/6); # G. C. Greubel, Oct 30 2019
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CROSSREFS
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Apart from initial terms, one more than the tetrahedral numbers A000292.
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KEYWORD
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easy,nonn
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AUTHOR
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Klaus Strassburger (strass(AT)ddfi.uni-duesseldorf.de), Dec 22 1999
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STATUS
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approved
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