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A239205
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Numbers k such that sigma(k) divides the sum of the numbers x coprime to k, with x <= k.
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3
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28, 140, 190, 248, 420, 477, 496, 672, 840, 910, 1428, 1488, 1547, 1638, 1848, 1892, 2295, 2398, 2480, 2660, 3417, 3472, 3515, 3640, 3720, 4064, 4095, 4590, 4641, 4655, 4845, 5456, 5655, 5995, 6200, 6678, 6888, 6944, 7030, 7440, 8008, 8128, 8190, 9214, 9282
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OFFSET
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1,1
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COMMENTS
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Numbers k such that sigma(k) | k*phi(k)/2.
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LINKS
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EXAMPLE
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28 is a term since 28*phi(28)/2 = 168, sigma(28) = 56 and 168 / 56 = 3.
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MAPLE
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with(numtheory); P:=proc(q) local a, n;
for n from 1 to q do a:=n*phi(n)/2; if type(a/sigma(n), integer) then print(n);
fi; od; end: P(10^6);
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MATHEMATICA
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Select[Range[10^4], Divisible[# * EulerPhi[#]/2, DivisorSigma[1, #]] &] (* Amiram Eldar, Mar 03 2020 *)
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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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