OFFSET
1,1
COMMENTS
EXAMPLE
10 is in the sequence because sigma(10) - 1 = 18 - 1 = 17 and sigma(phi(10)) = sigma(4) = 7 (both primes).
MATHEMATICA
Select[Range[10^6], And[PrimeQ[DivisorSigma[1, #] - 1], PrimeQ@ DivisorSigma[1, EulerPhi@ #]] &] (* Michael De Vlieger, Mar 17 2016 *)
PROG
(PARI) isok(n) = isprime(sigma(n)-1) && isprime(sigma(eulerphi(n))); \\ Michel Marcus, Mar 17 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
Jaroslav Krizek, Mar 16 2016
EXTENSIONS
a(35)-a(41) from Giovanni Resta, Apr 10 2016
STATUS
approved