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A239364
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Numbers n such that (n^2-4)/10 is a square.
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2
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38, 1442, 54758, 2079362, 78960998, 2998438562, 113861704358, 4323746327042, 164188498723238, 6234839205156002, 236759701297204838, 8990633810088627842, 341407325082070653158, 12964487719308596192162, 492309126008644584648998, 18694782300609185620469762
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OFFSET
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1,1
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COMMENTS
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Values of x satisfying the Pellian equation x^2 - 10*y^2 = 4.
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LINKS
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FORMULA
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a(n) = (19+6*sqrt(10))^(-n)+(19+6*sqrt(10))^n.
a(n) = 38*a(n-1)-a(n-2).
G.f.: -2*x*(x-19) / (x^2-38*x+1).
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EXAMPLE
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1442 is in the sequence because (1442^2-4)/10 = 207936 = 456^2.
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MATHEMATICA
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LinearRecurrence[{38, -1}, {38, 1442}, 30] (* Harvey P. Dale, Dec 19 2014 *)
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PROG
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(PARI) Vec(-2*x*(x-19)/(x^2-38*x+1) + O(x^100))
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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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