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A121690
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G.f.: A(x) = Sum_{k>=0} x^k * (1+x)^(k*(k+1)/2).
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5
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1, 1, 2, 4, 10, 27, 81, 262, 910, 3363, 13150, 54135, 233671, 1053911, 4951997, 24177536, 122381035, 640937746, 3466900453, 19337255086, 111057640382, 655892813805, 3978591077096, 24760700544301, 157941950878839
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OFFSET
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0,3
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COMMENTS
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a(n) is the number of length n permutations that simultaneously avoid the bivincular patterns (123,{2},{}) and (132,{},{2}). - Christian Bean, Jun 03 2015
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LINKS
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FORMULA
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a(n) = Sum_{k=0..n} C(k*(k+1)/2,n-k).
a(n) = A131338(n+1, n*(n+1)/2 + 1) for n>=0, where triangle A131338 starts with a '1' in row 0 and then for n>0 row n consists of n '1's followed by the partial sums of the prior row. - Paul D. Hanna, Aug 30 2007
Let q = (1+x), then g.f. A(x) equals the continued fraction:
A(x) = 1/(1 - q*x/(1 - (q^2-q)*x/(1 - q^3*x/(1 - (q^4-q^2)*x/(1 - q^5*x/(1- (q^6-q^3)*x/(1 - q^7*x/(1 - (q^8-q^4)*x/(1 - ...)))))))))
due to an identity of a partial elliptic theta function.
(End)
G.f.: Sum_{n>=0} x^n * Product_{k=1..n} (1 - x*(1+x)^(2*k-2))/(1 - x*(1+x)^(2*k-1)). - Paul D. Hanna, Mar 21 2018
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MATHEMATICA
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Table[Sum[Binomial[k*(k+1)/2, n-k], {k, 0, n}], {n, 0, 30}] (* Vaclav Kotesovec, Jun 03 2015 *)
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PROG
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(PARI) a(n) = sum(k=0, n, binomial(k*(k+1)/2, n-k))
for(n=0, 30, print1(a(n), ", "))
(PARI) {a(n)=polcoeff(sum(m=0, n, x^m*prod(k=1, m, (1 - x*(1+x)^(2*k-2))/(1 - x*(1+x)^(2*k-1) + x*O(x^n)))), n)}
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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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