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A204520
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Numbers such that floor(a(n)^2 / 5) is a square.
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19
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0, 1, 2, 3, 7, 9, 18, 47, 123, 161, 322, 843, 2207, 2889, 5778, 15127, 39603, 51841, 103682, 271443, 710647, 930249, 1860498, 4870847, 12752043, 16692641, 33385282
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OFFSET
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1,3
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COMMENTS
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Also: Numbers whose square, with its last base-5 digit dropped, is again a square. (For the three initial terms whose squares have only one digit in base 5, it is then understood that this yields zero.)
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LINKS
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FORMULA
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Empirical g.f.: -x^2*(x+1)*(3*x^6 + 4*x^5 + 14*x^4 - 5*x^3 - 2*x^2 - x-1) / ((x^4 - 4*x^2 - 1)*(x^4 + 4*x^2 - 1)). - Colin Barker, Sep 15 2014
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PROG
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(PARI) b=5; for(n=0, 2e9, issquare(n^2\b) && print1(n", "))
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CROSSREFS
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Cf. A031149, A055812, A204502, A204514, A204516, A204518 and A004275, A001075, A001541 for the analog in bases 10,...,6 and 4, 3, 2.
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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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