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A068474
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Numbers k such that phi((prime(k)-1)/2) = sigma(k).
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1
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19, 47, 77, 79, 115, 177, 191, 267, 280, 297, 340, 398, 406, 446, 470, 639, 839, 917, 1240, 1655, 1713, 1778, 1836, 2140, 2157, 4715, 5110, 5269, 6585, 7089, 7965, 8517, 8844, 9785, 10281, 12586, 15279, 16100, 16468, 16732, 17724, 18145, 18256, 20728
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OFFSET
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1,1
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LINKS
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MATHEMATICA
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Select[Range[2, 3000], EulerPhi[(Prime[#]-1)/2]==DivisorSigma[1, #]&] (* G. C. Greubel, Oct 13 2018 *)
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PROG
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(PARI) for(n=2, 45000, if(eulerphi((prime(n)-1)/2)==sigma(n), print1(n, ", ")))
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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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STATUS
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approved
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