OFFSET
1,1
COMMENTS
Observe that this sequence includes even numbers and for all primes p as (a phi-sum) an infinite number of solutions exist, like e.g. (2^w)*p, with 1+p-1=p Phi-sum over its factors.
LINKS
Michael De Vlieger, Table of n, a(n) for n = 1..10000
EXAMPLE
If n=2^a*3^b*5^c*7^d*11^e then prime-factor set is {2,3,5,7,11}. The totient function values of this set are {1,2,4,6,10} and the sum is 1+2+4+6+10=23.
MATHEMATICA
Select[Range@ 150, PrimeQ@ Total@ Map[EulerPhi@ # &, FactorInteger[#][[All, 1]]] &] (* Michael De Vlieger, Oct 26 2017 *)
PROG
(PARI) isok(n) = my(vp = factor(n)[, 1]); isprime(sum(i=1, #vp, eulerphi(vp[i]))); \\ Michel Marcus, Dec 19 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Jun 06 2000
STATUS
approved