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A062748
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Fourth column (r=3) of FS(3) staircase array A062745.
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13
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3, 9, 19, 34, 55, 83, 119, 164, 219, 285, 363, 454, 559, 679, 815, 968, 1139, 1329, 1539, 1770, 2023, 2299, 2599, 2924, 3275, 3653, 4059, 4494, 4959, 5455, 5983, 6544, 7139, 7769, 8435, 9138, 9879, 10659, 11479, 12340, 13243, 14189, 15179, 16214, 17295, 18423
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OFFSET
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0,1
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COMMENTS
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In the Frey-Sellers reference this sequence is called {(n+2) over 3}_{2}, n >= 0.
If X is an n-set and Y a fixed (n-3)-subset of X then a(n-3) is equal to the number of 3-subsets of X intersecting Y. - Milan Janjic, Aug 15 2007
Let A be the Hessenberg matrix of order n, defined by: A[1,j]=1, A[i,i]:=-1, A[i,i-1]=-1, and A[i,j]=0 otherwise. Then, for n>=6, a(n-6)=coeff(charpoly(A,x),x^(n-2)). - Milan Janjic, Jan 26 2010
For n>=4, a(n-4) is the number of permutations of 1,2,...,n, such that n-3 is the only up-point, or, the same, a(n-4) is up-down coefficient {n,4} (see comment in A060351). - Vladimir Shevelev, Feb 14 2014
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LINKS
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FORMULA
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a(n) = A062745(n+2, 3) = binomial(n+4, 3) - 1 = (n+1)*(n^2 + 8*n + 18)/3!.
G.f.: N(3;1, x)/(1-x)^4 with N(3;1, x) = 3 - 3*x + x^2, polynomial of the second row of A062746.
a(n) = sum of n successive triangular numbers A000217 starting from n=2.
a(n) = Sum[i(i+1)/2,{i=2..n}]. (End)
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EXAMPLE
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G.f. = 3 + 9*x + 19*x^2 + 34*x^3 + 55*x^4 + 83*x^5 + 119*x^6 + 164*x^7 + ...
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MAPLE
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a:=n->sum ((j+1)*j/2, j=2..n): seq(a(n), n=2..39); # Zerinvary Lajos, Dec 17 2006
seq(sum(sum(sum(1, k=0..l), l=0..m), m=1..n), n=1..38); # Zerinvary Lajos, Jan 26 2008
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MATHEMATICA
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k = 0; a = {}; Do[f = n(n + 1)/2; k = k + f; AppendTo[a, k], {n, 2, 100}]; a (* Artur Jasinski, Mar 14 2008 *)
LinearRecurrence[{4, -6, 4, -1}, {3, 9, 19, 34}, 40] (* Harvey P. Dale, Jan 13 2019 *)
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PROG
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(PARI) {a(n) = binomial(n+4, 3) - 1}; /* Michael Somos, Jan 28 2018 */
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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