A165210 Primes of the form (6^m - 1)/5.

7, 43, 55987, 7369130657357778596659, 3546245297457217493590449191748546458005595187661976371

Offset: 1

Author

Rick L. Shepherd, Sep 07 2009

Keywords

• nonn

Comments

Prime repunits in base 6 whose representation consists of m 1's. The exponents m are in A004062. a(5) and a(6) have 55 and 99 decimal digits, respectively.

Examples

			a(2) = (6^A004062(2) - 1)/5 = (6^3 - 1)/5 = 215/5 = 43, which is 111_6.
		

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..9

Crossrefs

Cf. A004062, A003464, A085104, A000668, A076481, A086122, A102170, A004022.

Programs

Magma

[a: n in [1..200] | IsPrime(a) where a is  (6^n-1) div 5 ]; // Vincenzo Librandi, Dec 09 2011

Mathematica

Select[Table[(6^n-1)/5, {n,0,2000}], PrimeQ] (* Vincenzo Librandi, Dec 09 2011 *)

Formula

a(n) = (6^A004062(n) - 1)/5.