A119739 Numbers k such that 7^k + 3 is semiprime.

1, 3, 31, 47, 83, 1255, 39015

Offset

1,2

Comments

Or (7^k + 3)/2 is prime.

a(7) > 18000. - Jinyuan Wang, Mar 04 2020

a(8) > 120000. - Tyler Busby, Feb 12 2023

Links

Table of n, a(n) for n=1..7.

Example

7^1 + 3 = 19 = 2*5,
7^3 + 3 = 346 = 2*173,
7^31 + 3 = 2*78887691017422903307521373,
7^47 + 3 = 2*2621669158378151817230729359430975727773,
7^83 + 3 = 2*6951960974910262341699296037696359556850100616048572362472005937832173.

Mathematica

Do[ If[ PrimeQ[(7^n + 3)/2], Print@n], {n, 1, 11501, 2}] (* Robert G. Wilson v, Jun 14 2006 *)

Prog

(PARI) is(k) = ispseudoprime((7^k+3)/2); \\ Jinyuan Wang, Mar 04 2020

Crossrefs

Sequence in context: A078330 A107210 A256473 * A163579 A290401 A341928

Adjacent sequences: A119736 A119737 A119738 * A119740 A119741 A119742

Keyword

nonn,more,less

Author

Zak Seidov, Jun 14 2006

Extensions

a(6) from Robert G. Wilson v, Jun 14 2006

a(7) from Tyler Busby, Feb 12 2023

Status

approved