OFFSET
1,1
COMMENTS
If n/2 is an odd prime and n+1 is prime then n is in the sequence, the proof is easy. 8,188,285,902,2013,... are terms of the sequence which they aren't of such form. This sequence is a subsequence of A066198.
If p is an odd Sophie Germain prime then 2*p is in the sequence. There is no term of the sequence which is of the form 2*p where p is prime and p isn't Sophie Germain prime. A244438 gives terms of the sequence which isn't of the form 2*p where p is prime. - Farideh Firoozbakht, Aug 14 2014
LINKS
Jens Kruse Andersen, Table of n, a(n) for n = 1..10000
EXAMPLE
10 is in the sequence because phi(10) + sigma(10) = 4 + 18 = 22 and phi(11) + sigma(11) = 10 + 12 = 22 also.
12 is not in the sequence because phi(12) + sigma(12) = 4 + 28 = 32 but phi(13) + sigma(13) = 12 + 14 = 26.
MATHEMATICA
Select[Range[2600], DivisorSigma[1, # ]+EulerPhi[ # ]==DivisorSigma[1, #+1]+EulerPhi[ #+1]&]
PROG
(PARI)
for(n=1, 10^4, s=eulerphi(n)+sigma(n); if(s==eulerphi(n+1)+sigma(n+1), print1(n, ", "))) /* Derek Orr, Aug 14 2014*/
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Farideh Firoozbakht, Nov 01 2008
STATUS
approved