%I #25 Feb 19 2021 10:29:10

%S 2,5,37,101,257,401,4801,12101,22501,25537,25601,31249,33857,160001,

%T 217157,401957,404497,476101,512657,583697,1020101,1270417,1322501,

%U 1503377,1674437,1943237,2005057,2016401,2056357,2689601,2755601,2842597,3686401,3920401

%N Primes p such that sigma(p-1)+sigma(p+1) is prime.

%H David A. Corneth, <a href="/A067464/b067464.txt">Table of n, a(n) for n = 1..10000</a> (first 150 terms from Hagen v. Eitzen)

%e 401 is here as 401 is prime and sigma(401 - 1) + sigma(401 + 1) = 961 + 816 = 1777 which is prime. - _David A. Corneth_, Feb 17 2021

%t Select[Prime[Range[300000]],PrimeQ[DivisorSigma[1,#-1]+DivisorSigma[ 1, #+1]]&] (* _Harvey P. Dale_, Jul 13 2018 *)

%o (PARI) isok(p) = isprime(p) && isprime(sigma(p-1)+sigma(p+1)); \\ _Michel Marcus_, Feb 17 2021

%o (PARI) upto(n) = {my(res = List()); for(i = 1, sqrtint(n + 1), if(isprime(2*i^2 - 1) && isprime(sigma(2*i^2-2) + sigma(2*i^2)) && 2*i^2 - 1 <= n, listput(res, 2*i^2 - 1); ); if(isprime(i^2 + 1) && isprime(sigma(i^2) + sigma(i^2 + 2)), listput(res, i^2 + 1); ) ); Set(res) } \\ _David A. Corneth_, Feb 17 2021

%Y Cf. A028982.

%K nonn

%O 1,1

%A _Benoit Cloitre_, Feb 23 2002

%E More terms from _Sascha Kurz_, Mar 18 2002