A256444 Numbers n such that sigma(n) = 2*(phi(n-1)+1).

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

Links

Table of n, a(n) for n=1..7.

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

Cf. A000010, A000203, A256439.

Sequence in context: A266165 A281622 A256439 * A032619 A193066 A193070

Adjacent sequences: A256441 A256442 A256443 * A256445 A256446 A256447

Keyword

nonn,more

Author

Jaroslav Krizek, Mar 31 2015

Extensions

a(7) from Giovanni Resta, Jul 13 2015

Status

approved