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A111247 Numbers k such that (31*10^k - 121) / 9 is prime. 0
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 (list; graph; refs; listen; history; text; internal format)
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 there because 3444431 is prime.

8 is there 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: A290213 A058775 A103023 * A045656 A129342 A045654

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

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Last modified March 24 19:48 EDT 2019. Contains 321448 sequences. (Running on oeis4.)