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A239205 Numbers k such that sigma(k) divides the sum of the numbers x coprime to k, with x <= k. 2
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Numbers k such that sigma(k) | k*phi(k)/2.

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..500 from Paolo P. Lava)

EXAMPLE

28 is a term since 28*phi(28)/2 = 168, sigma(28) = 56 and 168 / 56 = 3.

MAPLE

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);

MATHEMATICA

Select[Range[10^4], Divisible[# * EulerPhi[#]/2, DivisorSigma[1, #]] &] (* Amiram Eldar, Mar 03 2020 *)

CROSSREFS

Cf. A000010, A000203, A238232, A239208.

Sequence in context: A187047 A074233 A201095 * A050973 A095301 A201102

Adjacent sequences:  A239202 A239203 A239204 * A239206 A239207 A239208

KEYWORD

nonn,easy

AUTHOR

Paolo P. Lava, Mar 12 2014

STATUS

approved

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Last modified July 14 18:26 EDT 2020. Contains 335729 sequences. (Running on oeis4.)