A092587 - OEIS

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.

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

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OFFSET

1,1

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

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

CROSSREFS

Cf. A000010, A000203, A033632, A065395, A092584-A092588.

Sequence in context: A299380 A352159 A266956 * A247457 A015787 A322489

Adjacent sequences: A092584 A092585 A092586 * A092588 A092589 A092590

KEYWORD

nonn

AUTHOR

Labos Elemer, Mar 01 2004

STATUS

approved