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A256444
Numbers n such that sigma(n) = 2*(phi(n-1)+1).
2
3, 5, 17, 26, 257, 65537, 10866583226
OFFSET
1,1
COMMENTS
Subsequence of A256439. Supersequence of Fermat primes (A019434).
a(8) > 10^13. - Giovanni Resta, Jul 13 2015
EXAMPLE
17 is in the sequence because sigma(17) = 18 = 2*(phi(16-1)+1) = 2*9.
MATHEMATICA
Select[Range@ 100000, DivisorSigma[1, #] == 2 (EulerPhi[# - 1] + 1) &] (* Michael De Vlieger, Mar 31 2015 *)
PROG
(Magma) Set(Sort([n: n in [2..1000000] | SumOfDivisors(n) / (EulerPhi(n-1) + 1) eq 2 ]))
(PARI) first(m)={ my(v=vector(m), i, r); r=0; for(i=1, m, until(sigma(r)===2*(eulerphi(r-1)+1), r++); v[i]=r; print1(r, ", "); ); v; } Anders Hellström, Jul 29 2015
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Jaroslav Krizek, Mar 31 2015
EXTENSIONS
a(7) from Giovanni Resta, Jul 13 2015
STATUS
approved