A097652 - OEIS

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A097652

Numbers n such that n=phi(phi(n)+sigma(n)) and n is not of the form 2*p where p is a Sophie Germain odd prime.

1, 2, 20, 48, 180, 208, 864, 1120, 1368, 3552, 58320, 76416, 79968, 95488, 107520, 338688, 570240, 595968, 653184, 1347840, 5199552, 7918848, 14592000, 93699072, 159138176, 167078784, 246688000, 281640960, 314548224, 323985408, 338411520, 347578368, 352002048

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OFFSET

1,2

COMMENTS

It is obvious that if n=2*p where p is a Sophie Germain odd prime then n=phi(phi(n)+sigma(n)). This sequence is a subsequence of A097646. Except for the first term all terms of this sequence are even. There is no other term up to 3*10^7.

LINKS

Table of n, a(n) for n=1..33.

C. K. Caldwell, The Prime Glossary, Sophie Germain prime.

EXAMPLE

14592000 is in the sequence because 14592000=2*7296000, 7296000 is not a Sophie Germain odd prime and phi(phi(14592000)+sigma(14592000)) =14592000.

MATHEMATICA

Do[If[(!PrimeQ[n/2]||!PrimeQ[n+1])&&n==EulerPhi[EulerPhi[n]+ DivisorSigma[1, n]], Print[n]], {n, 30000000}]

CROSSREFS

Cf. A097646, A005384.

Sequence in context: A266842 A254542 A202602 * A261838 A349503 A225065

Adjacent sequences: A097649 A097650 A097651 * A097653 A097654 A097655

KEYWORD

nonn

AUTHOR

Farideh Firoozbakht, Sep 09 2004

EXTENSIONS

a(24)-a(33) from Donovan Johnson, Jan 18 2012

STATUS

approved