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A067651 Numbers n such that sigma(n) = phi(prime(n)-1). 2
1, 7, 19, 47, 115, 177, 280, 297, 340, 406, 470, 930, 1240, 1608, 1836, 2232, 2380, 2484, 4326, 8360, 33864, 166152, 216216, 220248, 299250, 394500 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

No further term below 4*10^6.

No further terms below 10^8. - Julien Kluge, Jul 08 2016

LINKS

Table of n, a(n) for n=1..26.

EXAMPLE

sigma(19) = 20, prime(19) = 67, phi(66) = 20, hence 20 is a term.

MATHEMATICA

Select[Range[1, 100000],

DivisorSigma[1, #] == EulerPhi[Prime[#] - 1] &] (* Julien Kluge, Jul 08 2016 *)

PROG

(PARI) m=400000; for(n=1, m, if(sigma(n)==eulerphi(prime(n)-1), print1(n, ", ")))

CROSSREFS

Cf. A107815, A107816.

Sequence in context: A146403 A000491 A097039 * A090025 A003232 A018728

Adjacent sequences:  A067648 A067649 A067650 * A067652 A067653 A067654

KEYWORD

nonn,more

AUTHOR

Benoit Cloitre, Feb 23 2002

EXTENSIONS

More terms from Klaus Brockhaus, Mar 02 2002, revised May 24 2005

STATUS

approved

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Last modified September 26 18:59 EDT 2021. Contains 347670 sequences. (Running on oeis4.)