A111247 Numbers k such that (31*10^k - 121) / 9 is prime.

2, 6, 8, 21, 66, 84, 86, 89, 93, 104, 170, 249, 273, 413, 824, 1392, 1589, 1679, 3125, 3185, 3482, 4089, 4772, 5138, 5738, 54761, 86364

Offset

1,1

Comments

Variant of repunit primes: each value is 3444...44431, with k-2 4's.

a(28) > 2*10^5. - Robert Price, Jun 07 2016

Links

Table of n, a(n) for n=1..27.

Makoto Kamada, Factorization of near-repdigit-related numbers.

Makoto Kamada, Search for 34w31.

Example

6 is a term because 3444431 is prime.
8 is a term because 344444431 is prime.

Mathematica

Select[Range[1200], PrimeQ[(31*10^# - 121)/9]&] (* Vladimir Joseph Stephan Orlovsky, Feb 01 2012 *)

Prog

(PARI) is(n)=ispseudoprime((31*10^n - 121)/9) \\ Charles R Greathouse IV, Jun 08 2016

Crossrefs

Sequence in context: A058775 A103023 A341732 * A045656 A343256 A129342

Adjacent sequences: A111244 A111245 A111246 * A111248 A111249 A111250

Keyword

nonn,more

Author

Joao da Silva (zxawyh66(AT)yahoo.com), Oct 31 2005

Extensions

Edited by Don Reble, Nov 09 2005

a(16) - a(23) from Vladimir Joseph Stephan Orlovsky, Feb 01 2012

a(24) - a(27) from Robert Price, Jun 07 2016

Status

approved