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A026379
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a(n) = number of integer strings s(0),...,s(n) counted by array T in A026374 that have s(n)=3; also a(n) = T(2n-1,n-2).
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2
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1, 7, 39, 202, 1015, 5028, 24731, 121208, 593019, 2899335, 14173401, 69301422, 338990145, 1659037695, 8124085575, 39806373880, 195160896835, 957396540285, 4699409632805, 23080158080150, 113414575414245, 557601196738190
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OFFSET
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2,2
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LINKS
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FORMULA
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a(n) = [t^(n+1)]{(1+t)(1+3t+t^2)^(n-1)}. - Emeric Deutsch, Jan 30 2004
G.f.: -1/x+2-2*x^2/(10*x^2+sqrt(1-5*x)*sqrt(1-x)*(4*x-1)-7*x+1). - Vladimir Kruchinin, Aug 11 2015
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MAPLE
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sum(binomial(n, k)*binomial(2*k+1, k-1), k=0..n); n=0, 1, ... # N. J. A. Sloane
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MATHEMATICA
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Table[Sum[Binomial[n, k]Binomial[2k+1, k-1], {k, 0, n}], {n, 30}] (* Harvey P. Dale, Apr 28 2013 *)
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CROSSREFS
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KEYWORD
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nonn,nice,easy
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AUTHOR
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STATUS
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approved
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