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A190503
Numbers k such that sigma(phi(k)) divides sigma(k).
3
1, 2, 6, 12, 14, 22, 24, 28, 44, 46, 48, 56, 68, 87, 88, 92, 94, 96, 112, 118, 166, 174, 176, 184, 188, 192, 214, 224, 236, 332, 334, 352, 358, 362, 368, 376, 384, 390, 410, 428, 448, 454, 472, 526, 664, 668, 694, 704, 716, 718, 736, 752, 766, 768, 856, 896
OFFSET
1,2
COMMENTS
These numbers appear indirectly in A067740, which seeks the least k such that sigma(k)/sigma(phi(k)) = n. Most of these numbers are even. The odd terms (1, 87, 1257, 41559, 56679, ...) all appear to produce sigma(k)/sigma(phi(k)) = 1.
MATHEMATICA
Select[Range[1000], IntegerQ[DivisorSigma[1, #]/DivisorSigma[1, EulerPhi[#]]] &]
PROG
(PARI) is(k) = {my(f = factor(k), s = sigma(f), p = eulerphi(f)); !(s % sigma(p)); } \\ Amiram Eldar, May 17 2024
CROSSREFS
Cf. A000010 (phi), A000203 (sigma), A062402, A067740.
Cf. A033631 (k such that sigma(k)/sigma(phi(k)) = 1).
Cf. A066831 (k such that sigma(k) divides sigma(phi(k))).
Sequence in context: A333833 A057895 A111369 * A320149 A346305 A140760
KEYWORD
nonn
AUTHOR
T. D. Noe, May 11 2011
STATUS
approved