A238848 Smallest k such that k*n^3 - 1 is prime.

3, 1, 2, 2, 4, 2, 14, 7, 6, 2, 4, 4, 14, 3, 4, 2, 16, 4, 12, 9, 2, 5, 16, 2, 2, 3, 16, 6, 10, 4, 2, 4, 22, 2, 6, 3, 6, 10, 6, 3, 22, 5, 2, 3, 4, 2, 18, 4, 26, 10, 4, 5, 6, 2, 2, 7, 6, 2, 10, 5, 2, 9, 4, 2, 16, 3, 6, 9, 2, 3, 30, 5, 14, 6, 24, 5, 16, 5

Offset

1,1

Links

Harvey P. Dale, Table of n, a(n) for n = 1..1000

Example

a(1) = 3 because for k = 1, 1*(1^3) - 1 = 0 is not prime, for k = 2, 2*(1^3) - 1 = 1 is not prime, but for k = 3, 3*(1^3) - 1 = 2 is prime.

Mathematica

sk[n_]:=Module[{k=1, n3=n^3}, While[!PrimeQ[k*n3-1], k++]; k]; Array[sk, 80] (* Harvey P. Dale, Jan 04 2023 *)

Prog

(Python)
import sympy
from sympy import isprime
def f(n):
..for k in range(1, 10**3):
....if isprime(k*(n**3)-1):
......return k
n = 1
while n < 10**3:
..print(f(n))
..n += 1

Crossrefs

Cf. A231819, A053989.

Sequence in context: A125266 A002016 A282016 * A117494 A342913 A221490

Adjacent sequences: A238845 A238846 A238847 * A238849 A238850 A238851

Keyword

nonn

Author

Derek Orr, Mar 06 2014

Status

approved