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A204516
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Numbers such that floor(a(n)^2 / 7) is a square.
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20
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0, 1, 2, 3, 8, 16, 45, 127, 254, 717, 2024, 4048, 11427, 32257, 64514, 182115, 514088, 1028176, 2902413, 8193151, 16386302, 46256493, 130576328, 261152656, 737201475, 2081028097, 4162056194, 11748967107, 33165873224, 66331746448
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OFFSET
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1,3
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COMMENTS
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Or: Numbers whose square, with its last base-7 digit dropped, is again a square (where for the first 3 terms, dropping the digit is meant to yield zero).
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LINKS
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FORMULA
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G.f. = (x + 2*x^2 + 3*x^3 - 8*x^4 - 16*x^5 - 3*x^6 )/(1 - 16*x^3 + x^6).
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MATHEMATICA
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LinearRecurrence[{0, 0, 16, 0, 0, -1}, {0, 1, 2, 3, 8, 16, 45}, 30] (* or *) CoefficientList[Series[ (x+2x^2+3x^3-8x^4-16x^5-3x^6)/(1-16x^3+x^6), {x, 0, 30}], x] (* Harvey P. Dale, Apr 22 2023 *)
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PROG
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(PARI) b=7; for(n=0, 2e9, issquare(n^2\b) & print1(n", "))
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CROSSREFS
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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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