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A157535
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Half the number of length n integer sequences with sum zero and sum of squares 18.
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1
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1, 3, 18, 70, 285, 1533, 7084, 29604, 116385, 419815, 1385142, 4278066, 12678757, 36517950, 102364200, 278451568, 733599079, 1872496417, 4639365470, 11183927650, 26287242751, 60341512253, 135432776524, 297482865100
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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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[cache enabling] count(n,s,ss)->count(n,t,tt) where t=s mod n, q=(t-s)/n, tt=ss+2*q*s+n*q^2; count(n,t,tt)=sum[i^2<=tt]{ count(n-1,t-i,tt-i^2) }. a(n)=count(n,0,18)/2.
G.f.: -x^2*(434*x^16 +19697*x^15 +99303*x^14 -116957*x^13 -153862*x^12 +406284*x^11 -362744*x^10 +160940*x^9 -16710*x^8 -25990*x^7 +20878*x^6 -9354*x^5 +3002*x^4 -728*x^3 +132*x^2 -16*x +1) / (x -1)^19. - Colin Barker, Apr 08 2014
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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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