A119720 Numbers k such that 7^k + 2 is semiprime.

1, 2, 6, 9, 12, 14, 26, 33, 44, 54, 126, 141, 830, 2436, 3534, 3846, 4152, 4460, 4974, 13160, 13200, 13832, 21996, 28065

Offset

1,2

Comments

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

a(25) > 77777. - Tyler Busby, Feb 12 2023

Links

Table of n, a(n) for n=1..24.

Example

12 is a term because 7^12 + 2 = 13841287203 = 3*4613762401 and 4613762401 is prime.

Mathematica

Select[Range[2100], PrimeQ[(7^# + 2)/3] &] (* Stefan Steinerberger, Jul 22 2006 *)

Crossrefs

Sequence in context: A153043 A171504 A287445 * A173978 A000134 A120701

Adjacent sequences: A119717 A119718 A119719 * A119721 A119722 A119723

Keyword

nonn,more

Author

Zak Seidov, Jun 13 2006

Extensions

More terms from Stefan Steinerberger and Joshua Zucker, Jun 21 2006

a(14) from Herman Jamke (hermanjamke(AT)fastmail.fm), Mar 30 2008

a(15)-a(22) from Sean A. Irvine, Dec 16 2009

a(23)-a(24) from Tyler Busby, Feb 12 2023

Status

approved