OFFSET
1,1
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..1000
EXAMPLE
Includes all odd primes and some composites; e.g., 21 and 25, since sigma(21) + phi(21) = 32 + 12 = 44 = 2*2*11; sigma(25) + phi(25) = 31 + 20 = 51 = 3*17.
MATHEMATICA
Select[ Range[ 1, 100 ], Length[ FactorInteger[ DivisorSigma[ 1, # ]+EulerPhi[ # ] ] ]==2& ]
Select[Range[500], PrimeNu[EulerPhi[#] + DivisorSigma[1, #]] == 2 &] (* G. C. Greubel, May 08 2017 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Jan 17 2002
EXTENSIONS
Edited by Dean Hickerson, Jan 20 2002
STATUS
approved