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A073541
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Numbers k such that 1/(1/sigma(k) + 1/sigma(k+1)) is an integer.
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2
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5, 14, 54, 55, 56, 58, 70, 87, 88, 89, 114, 118, 123, 125, 178, 201, 204, 206, 220, 246, 354, 358, 417, 419, 459, 478, 500, 534, 549, 594, 604, 620, 646, 654, 663, 715, 718, 753, 834, 835, 838, 894, 957, 1006, 1018, 1106, 1194, 1239, 1253, 1318, 1334, 1364
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OFFSET
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1,1
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LINKS
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EXAMPLE
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1/sigma(54) + 1/sigma(55) = 1/120 + 1/72 = 1/45 so 54 is in the sequence.
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MATHEMATICA
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Select[Range[1400], IntegerQ[1/(1/DivisorSigma[1, # ]+1/DivisorSigma[1, #+1])]&]
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PROG
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(PARI) isok(k) = denominator(1/(1/sigma(k) + 1/sigma(k+1))) == 1; \\ Michel Marcus, May 13 2022
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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