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A092587
Numbers k such that sigma(phi(k))-phi(sigma(k)) is nonzero and divisible by phi(k), that is A065395(k)/A000010(k) is a nonzero integer.
4
2, 18, 21, 99, 133, 151, 175, 183, 350, 366, 449, 450, 477, 532, 581, 645, 702, 843, 1072, 1253, 1346, 1508, 1645, 1833, 2085, 2097, 2150, 2421, 3668, 3950, 4223, 4312, 4453, 5264, 6601, 6853, 7128, 7423, 7622, 7713, 8325, 9028, 9364, 9707, 10820
OFFSET
1,1
LINKS
EXAMPLE
(sigma(phi(x))-phi(sigma(x)))/phi(x) quotient equals -3 for x=450, -2 for x=18, -1 for x=2, 1 for x=21, 2 for x=99, 3 for x=4223.
MATHEMATICA
fs[x_] := EulerPhi[DivisorSigma[1, x]] sf[x_] := DivisorSigma[1, EulerPhi[x]] {t=Table[0, {60}], j=1}; Do[s=(sf[n]-fs[n])/EulerPhi[n]; If[ !Equal[s, 0]&&IntegerQ[s], Print[n]; t[[j]]=n; j=j+1], {n, 2, 1000000}] t
KEYWORD
nonn
AUTHOR
Labos Elemer, Mar 01 2004
STATUS
approved