OFFSET
1,1
LINKS
David A. Corneth, Table of n, a(n) for n = 1..10000 (first 150 terms from Hagen v. Eitzen)
EXAMPLE
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
MATHEMATICA
Select[Prime[Range[300000]], PrimeQ[DivisorSigma[1, #-1]+DivisorSigma[ 1, #+1]]&] (* Harvey P. Dale, Jul 13 2018 *)
PROG
(PARI) isok(p) = isprime(p) && isprime(sigma(p-1)+sigma(p+1)); \\ Michel Marcus, Feb 17 2021
(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
CROSSREFS
KEYWORD
nonn
AUTHOR
Benoit Cloitre, Feb 23 2002
EXTENSIONS
More terms from Sascha Kurz, Mar 18 2002
STATUS
approved