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A263946
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Positive integers n such that (n+52)^3 - n^3 is a square.
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8
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26, 2626, 132522, 6624722, 331104826, 16548617826, 827099787722, 41338440769522, 2066094938689626, 103263408493713026, 5161104329746962922, 257951953078854434322, 12892436549612974754426, 644363875527569883288226, 32205301339828881189658122
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OFFSET
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1,1
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LINKS
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FORMULA
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a(n) = 51*a(n-1)-51*a(n-2)+a(n-3) for n>3.
G.f.: 26*x*(3*x^2-50*x-1) / ((x-1)*(x^2-50*x+1)).
a(n) = 26*(-6-(6+sqrt(39))*(25+4*sqrt(39))^(-n)+(-6+sqrt(39))*(25+4*sqrt(39))^n)/6. - Colin Barker, Mar 03 2016
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EXAMPLE
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26 is in the sequence because (26+52)^3 - 26^3 = 676^2.
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MATHEMATICA
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LinearRecurrence[{51, -51, 1}, {26, 2626, 132522}, 20] (* Harvey P. Dale, Feb 05 2019 *)
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PROG
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(PARI) Vec(26*x*(3*x^2-50*x-1)/((x-1)*(x^2-50*x+1)) + O(x^30))
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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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