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A074875 Numbers n such that sigma(sigma(n)-n) = phi(n). 1
2, 21, 51, 369, 3051, 3783, 5757, 6477, 6897, 7929, 15639, 15925, 20967, 33003, 50739, 58797, 73917, 75627, 118405, 121975, 131979, 170973, 219447, 291369, 310137, 311675, 347337, 365679, 396807, 404559, 437255, 442599, 550719, 558429 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Donovan Johnson, Table of n, a(n) for n = 1..300

EXAMPLE

sigma(sigma(21) - 21) = sigma(32 - 21) = sigma(11) = 12 = phi(21), so 21 is a term of the sequence.

MATHEMATICA

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

CROSSREFS

Sequence in context: A330201 A042455 A245546 * A231807 A351118 A097718

Adjacent sequences:  A074872 A074873 A074874 * A074876 A074877 A074878

KEYWORD

nonn

AUTHOR

Joseph L. Pe, Sep 12 2002

EXTENSIONS

a(19)-a(34) from Donovan Johnson, May 13 2010

STATUS

approved

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Last modified October 7 05:49 EDT 2022. Contains 357270 sequences. (Running on oeis4.)