OFFSET
1,1
EXAMPLE
If k = 2, u(1) = 2 is prime, and no lesser number has this property, thus 2 is a term.
If k = 11, u(1) = 11, u(2) = 11 + 2! = 13, u(3) = 13 - 3! = 7 and u(4) = 7 + 4! = 31 are primes, and no lesser number has this property, thus 11 is a term.
PROG
(PARI)
card(k)=my(s=k, c=2, p=1); while(isprime(s), p*=c; s+=if(c%2, -p, p); c++); c-=2
rec(kk=10^9)=my(r=0); forprime(k=1, kk, x=card(k); if(x>r, if(r>0, print1(", ")); print1(k); r=x))
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Jean-Marc Rebert, Nov 23 2022
STATUS
approved