A067198 - OEIS

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A067198

Numbers k such that sigma(k) = phi(k) + phi(k-1).

5, 7, 77, 6293, 12943, 23495, 30191, 99695, 147407, 240425, 258401, 535601, 1103825, 1570145, 1637867, 1972607, 2219135, 2241281, 2970517, 3785951, 5612321, 5917003, 6215951, 9249241, 9281899, 12496429, 14779531, 25739701, 30367321, 33522251, 33852841

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

LINKS

Table of n, a(n) for n=1..31.

EXAMPLE

sigma(5) = 6 = 4 + 2 = phi(5) + phi(5-1), so 5 is a term of the sequence.

MATHEMATICA

Select[Range[2, 10^5], EulerPhi[ # ] + EulerPhi[ # - 1] == DivisorSigma[1, # ] &]

PROG

(PARI) is(k) = if(k < 2, 0, my(f = factor(k)); eulerphi(k-1) + eulerphi(f) == sigma(f)); \\ Amiram Eldar, Nov 10 2024

CROSSREFS