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A038344 Numbers k such that phi(k) + sigma(k) is a prime. 8
1, 8, 9, 32, 36, 50, 100, 225, 242, 484, 512, 578, 729, 800, 900, 1089, 1156, 1250, 1936, 2025, 2048, 2304, 2312, 2601, 2916, 3025, 3872, 4418, 6400, 7225, 7744, 8192, 8464, 8836, 9216, 10000, 12800, 14400, 20000, 20736, 21609, 26896, 27556, 31684, 32768, 33856, 34322 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Indices k such that A065387(k) is a prime number. - R. J. Mathar, Aug 26 2009

All terms are squares or twice squares (A028982). - Donovan Johnson, Sep 27 2013

LINKS

Donovan Johnson, Table of n, a(n) for n = 1..1000

FORMULA

{k: A000203(k) + A000010(k) in A000040}. - R. J. Mathar, Aug 26 2009

EXAMPLE

a(2) = 8 because phi(8) + sigma(8) = 19.

MATHEMATICA

Select[Range[0, 40000], PrimeQ[DivisorSigma[1, #] + EulerPhi[#]] &] (* Vincenzo Librandi, Jul 22 2016 *)

PROG

(PARI) isok(n) = isprime(eulerphi(n) + sigma(n)); \\ Michel Marcus, Sep 27 2013

(PARI) v=vector(1000); c=0; for(j=1, 12105, m=j^2; if(isprime(eulerphi(m)+sigma(m)), c++; v[c]=m)); for(j=1, 8559, m=2*j^2; if(isprime(eulerphi(m)+sigma(m)), c++; v[c]=m)); v=vecsort(v); for(n=1, 1000, write("b038344.txt", n " " v[n])) \\ Donovan Johnson, Sep 27 2013

(MAGMA) [n: n in [1..40000] | IsPrime(EulerPhi(n)+DivisorSigma(1, n))]; // Vincenzo Librandi, Jul 22 2016

CROSSREFS

Cf. A000010, A000027, A000040, A000203, A028982.

Sequence in context: A225364 A304922 A181900 * A306162 A050771 A120776

Adjacent sequences:  A038341 A038342 A038343 * A038345 A038346 A038347

KEYWORD

nonn

AUTHOR

Felice Russo

EXTENSIONS

More terms from Olivier Gérard

STATUS

approved

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Last modified September 28 06:17 EDT 2021. Contains 347703 sequences. (Running on oeis4.)