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
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
KEYWORD
nonn
AUTHOR
EXTENSIONS
More terms from Olivier Gérard
STATUS
approved