OFFSET
1,1
COMMENTS
These primes may be part of Cunningham chains longer than three terms. It seems the two operators are never mixed, except for 3, 5 and 7: -for 3, we have: 2 through <2p-1> -> 3 through <2p+1> -> 7 -for 5: 3 <2p-1> -> 5 <2p+1> -> 11 -for 7: 3 <2p+1> -> 7 <2p-1> -> 13
For p > 7, such a mixed chain with p in the middle is impossible because the number 3 would be a nontrivial factor of either the smallest or the largest term. - Jeppe Stig Nielsen, May 05 2019
Primes (excluding 2 and 7) that divide more than one semiprime triangular number A068443. - Jeppe Stig Nielsen, May 05 2019
LINKS
Chris Caldwell's Prime Glossary, Cunningham chains.
EXAMPLE
a(3)=11 is here because 5->11->23 through <2p+1>;
a(4)=23 because 11->23->47 through <2p+1>;
a(5)=37 because 19->37->73 through <2p-1>.
PROG
Terms computed by Gilles Sadowski.
(PARI) forprime(p=3, 10^6, if(p%3==2, isprime((p-1)/2)&&isprime(2*p+1), isprime((p+1)/2)&&isprime(2*p-1))&&print1(p, ", ")) \\ Jeppe Stig Nielsen, May 05 2019
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Alexandre Wajnberg, Aug 31 2005
STATUS
approved