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%I #22 Sep 08 2022 08:46:16
%S 3,5,7,17,67,257,65537,8942223643
%N Primes p such that sigma(p-1) + phi(p-1) = (5p-7)/2.
%C Primes p such that A065387(p-1) = (5p-7)/2.
%C Fermat primes from A019434 are terms.
%C Prime terms from A270837.
%C a(9), if it exists, is larger than 10^13. - _Giovanni Resta_, Apr 10 2016
%e 17 is in the sequence because sigma(16)+phi(16) = 31+8 = 39 = (5*17-7)/2.
%t Select[Prime@ Range[10^4], 2 (DivisorSigma[1, # - 1] + EulerPhi[# - 1]) == 5 # - 7 &] (* _Michael De Vlieger_, Mar 24 2016 *)
%o (Magma) [n: n in[1..10^7] | IsPrime(n) and 2*(SumOfDivisors(n-1) + EulerPhi(n-1)) eq 5*n-7]
%o (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
%Y Cf. A000010, A000203, A065387, A270778, A270837.
%K nonn,more
%O 1,1
%A _Jaroslav Krizek_, Mar 22 2016
%E a(8) from _Michel Marcus_, Mar 23 2016
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