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A128049 Least number k>0 such that abs((3^k - (3-n)^k)/n) is prime, or 0 if no such prime exists. 2
0, 2, 3, 2, 2, 3, 0, 3, 2, 2, 3, 2, 0, 2, 3, 0, 3, 2, 0, 2, 37, 0, 3, 2, 0, 2, 1153, 0, 83, 2, 0, 3, 11, 0, 3, 2, 0, 2, 3, 0, 557, 19, 0, 2, 3, 0, 7, 2, 0, 2, 631, 0, 5, 2, 0, 3, 3, 0, 239, 2, 0, 5, 3, 0, 3, 2, 0, 2, 317, 0, 3, 103, 0, 2, 7, 0, 3, 2, 0, 2, 43 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

a(-n) = A128033(n).

a(3*n) = 0 except a(3) = a(9) = 2.

All positive terms are primes.

LINKS

Jinyuan Wang, Table of n, a(n) for n = 0..222

PROG

(PARI) a(n) = my(p=3); if(isprime(abs(n-6)), 2, if(n%3, while(!ispseudoprime((3^p-(3-n)^p)/n), p=nextprime(p+1)); p, 0)); \\ Jinyuan Wang, Nov 28 2020

CROSSREFS

Cf. A128033 (least number k>0 such that ((n+3)^k - 3^k)/n is prime), A028491 (numbers n such that (3^n - 1)/2 is prime).

Cf. A111010, A128024, A128025, A128026, A128027, A128028, A128029, A128030, A128031, A128032.

Sequence in context: A145390 A270026 A340703 * A104543 A054988 A143393

Adjacent sequences:  A128046 A128047 A128048 * A128050 A128051 A128052

KEYWORD

nonn

AUTHOR

Alexander Adamchuk, Feb 12 2007

EXTENSIONS

Name changed by Jinyuan Wang, Nov 28 2020

STATUS

approved

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Last modified June 17 00:17 EDT 2021. Contains 345080 sequences. (Running on oeis4.)