A270779 Primes p such that sigma(p-1) + phi(p-1) = (5p-7)/2.

3, 5, 7, 17, 67, 257, 65537, 8942223643

Offset

1,1

Comments

Primes p such that A065387(p-1) = (5p-7)/2.

Fermat primes from A019434 are terms.

Prime terms from A270837.

a(9), if it exists, is larger than 10^13. - Giovanni Resta, Apr 10 2016

Links

Table of n, a(n) for n=1..8.

Example

17 is in the sequence because sigma(16)+phi(16) = 31+8 = 39 = (5*17-7)/2.

Mathematica

Select[Prime@ Range[10^4], 2 (DivisorSigma[1, # - 1] + EulerPhi[# - 1]) == 5 # - 7 &] (* Michael De Vlieger, Mar 24 2016 *)

Prog

(Magma) [n: n in[1..10^7] | IsPrime(n) and 2*(SumOfDivisors(n-1) + EulerPhi(n-1)) eq 5*n-7]
(PARI) lista(nn) = forprime(p=2, nn, if (sigma(p-1) + eulerphi(p-1) == (5*p-7)/2, print1(p, ", "))); \\ Michel Marcus, Mar 23 2016

Crossrefs

Cf. A000010, A000203, A065387, A270778, A270837.

Sequence in context: A345529 A215684 A229132 * A350176 A248796 A247164

Adjacent sequences: A270776 A270777 A270778 * A270780 A270781 A270782

Keyword

nonn,more

Author

Jaroslav Krizek, Mar 22 2016

Extensions

a(8) from Michel Marcus, Mar 23 2016

Status

approved