OFFSET
1,1
COMMENTS
For all terms up to 10^12, sigma(n)+phi(n) is a product of 2 distinct primes. The only other possibility is that sigma(n)+phi(n) is a cube of a prime, for some n which is either a square or twice a square; does this occur? If not, then this sequence is contained in A067351.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
EXAMPLE
Includes all odd primes and some composites; e.g. 22 and 25, since sigma(22)+phi(22)=36+10=46=2*23 and sigma(25)+phi(25)=31+20=51=3*17.
MATHEMATICA
Select[ Range[ 1, 200 ], DivisorSigma[ 0, DivisorSigma[ 1, # ]+EulerPhi[ # ] ]==4& ]
PROG
(PARI) isok(n) = numdiv(sigma(n)+eulerphi(n)) == 4; \\ Michel Marcus, Aug 13 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Jan 17 2002
EXTENSIONS
Edited by Dean Hickerson, Jan 20 2002
STATUS
approved