OFFSET
1,2
COMMENTS
If n=2*p where p is a Sophie Germain odd prime, then n is in the sequence; the proof is obvious.
LINKS
K. D. Bajpai, Table of n, a(n) for n = 1..10000
C. K. Caldwell, The Prime Glossary, Sophie Germain prime.
EXAMPLE
22 is in the sequence because phi(22)=10, sigma(22)=36 and phi(10+36)=22.
MAPLE
with(numtheory):K:=proc()local n, a, c; c:=1; for n from 1 to 5000000 do;
a:=phi(phi(n)+ sigma(n)); if a=n then lprint(c, n); c:=c+1; fi; od; end:K(); # K. D. Bajpai, Jul 18 2013
MATHEMATICA
Do[If[n==EulerPhi[EulerPhi[n]+DivisorSigma[1, n]], Print[n]], {n, 2400}]
Select[Range[2500], EulerPhi[EulerPhi[#]+DivisorSigma[1, #]]==#&] (* Harvey P. Dale, Jul 06 2021 *)
PROG
(PARI) is(n)=sigma(n=factor(n))==eulerphi(eulerphi(n)) \\ Charles R Greathouse IV, Nov 27 2013
(Magma) [n: n in [1..2300] | n eq EulerPhi(EulerPhi(n) + DivisorSigma(1, n))]; // Vincenzo Librandi, Aug 22 2015
CROSSREFS
KEYWORD
nonn
AUTHOR
Farideh Firoozbakht, Sep 08 2004
STATUS
approved