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

3, 5, 17, 26, 257, 65537, 10866583226

Offset: 1

Author

Jaroslav Krizek, Mar 31 2015

Keywords

• nonn
• more

Comments

Subsequence of A256439. Supersequence of Fermat primes (A019434).

a(8) > 10^13. - Giovanni Resta, Jul 13 2015

Examples

			17 is in the sequence because sigma(17) = 18 = 2*(phi(16-1)+1) = 2*9.
		

Crossrefs

Cf. A000010, A000203, A256439.

Programs

Magma

Set(Sort([n: n in [2..1000000] | SumOfDivisors(n) / (EulerPhi(n-1) + 1) eq 2 ]));

Mathematica

Select[Range@ 100000, DivisorSigma[1, #] == 2 (EulerPhi[# - 1] + 1) &] (* Michael De Vlieger, Mar 31 2015 *)

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

Extensions

a(7) from Giovanni Resta, Jul 13 2015