OFFSET
1,1
COMMENTS
For n <= 10^9 the largest term is 186.
First occurrence of 2k, k=1,2,3,...: 1, 6, 15, 35, 39, 117, 1134, 199, 152, 362, ..., . - Robert G. Wilson v, Feb 08 2018
EXAMPLE
For n = 6, prime(6) = 13. The smallest even number k such that k * 13 + 1 is a prime number is k = 4, because 4 * 13 + 1 = 53 (not k = 2). So 4 is the sixth term.
MATHEMATICA
f[n_] := Block[{k = 2, p = Prime@ n}, While[ !PrimeQ[k*p -1] && !PrimeQ[k*p +1], k += 2]; k]; Array[f, 100] (* Robert G. Wilson v, Feb 08 2018 *)
PROG
(PARI) {
forprime(p=2, 100,
k=2;
while(!isprime(k*p-1)&&!isprime(k*p+1), k+=2);
print1(k", ");
)
}
CROSSREFS
KEYWORD
nonn
AUTHOR
Dimitris Valianatos, Feb 07 2018
STATUS
approved