A245516 The smallest odd number k such that k^n-2 is a prime number.

5, 3, 9, 3, 3, 3, 7, 7, 3, 21, 9, 7, 19, 5, 7, 39, 15, 61, 15, 19, 21, 3, 19, 17, 21, 5, 21, 7, 85, 17, 7, 21, 511, 27, 27, 59, 3, 19, 91, 45, 3, 29, 321, 65, 9, 379, 69, 125, 49, 5, 9, 45, 289, 341, 61, 89, 171, 171, 139, 21, 139, 75, 25, 49, 15, 51, 57, 175

Offset

1,1

Links

Zak Seidov, Table of n, a(n) for n = 1..200

Example

n=1, 3-2=1 is not prime, 5-2=3 is a prime number.  So a(1) = 5.
n=2, 3^2-2=7 is a prime number.  So a(2) = 3.
n=10, for k=3, 5, ..., 19, k^10-2 are all composite.  21^10-2 = 16679880978199 is a prime number.  So a(10) = 21.

Maple

A245516 := proc(n)
    for k from 1 by 2 do
        if isprime(k^n-2) then
            return k;
        end if;
    end do:
end proc:
seq(A245516(n), n=1..60) ;

Mathematica

Table[n = 1;
While[n = n + 2; cp = n^i - 2; ! PrimeQ[cp]]; n, {i, 1, 68}]

Crossrefs

Cf. A095303.

Sequence in context: A241624 A159275 A059031 * A073243 A134943 A105372

Adjacent sequences: A245513 A245514 A245515 * A245517 A245518 A245519

Keyword

nonn

Author

Lei Zhou, Jul 24 2014

Status

approved