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A280848 Numbers k such that (4*10^k + 173)/3 is prime. 0
0, 1, 2, 5, 7, 8, 12, 42, 80, 91, 138, 169, 1376, 1715, 3212, 6410, 8492, 9257, 10584, 11123, 13819, 17532, 20369, 22513, 56156, 69626, 71498, 83201, 155022, 197778 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

For k>1, numbers such that the digit 1 followed by k-2 occurrences of the digit 3 followed by the digits 91 is prime (see Example section).

a(31) > 2*10^5.

LINKS

Table of n, a(n) for n=1..30.

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

Makoto Kamada, Search for 13w91.

EXAMPLE

2 is in this sequence because (4*10^2 + 173) / 3 = 191 is prime.

Initial terms and primes associated:

a(1) = 0, 59;

a(2) = 1, 71;

a(3) = 2, 191;

a(4) = 5, 133391;

a(5) = 7, 13333391; etc.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[(4*10^# + 173) / 3] &]

CROSSREFS

Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.

Sequence in context: A294635 A032402 A299326 * A190900 A216572 A020674

Adjacent sequences:  A280845 A280846 A280847 * A280849 A280850 A280851

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Jan 09 2017

EXTENSIONS

a(29)-a(30) from Robert Price, Feb 16 2018

STATUS

approved

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Last modified October 1 01:07 EDT 2022. Contains 357127 sequences. (Running on oeis4.)