A287208 - OEIS

A287208

Numbers k such that 8*10^k - 13 is prime.

1, 2, 4, 7, 8, 13, 23, 28, 37, 40, 107, 132, 156, 290, 295, 653, 948, 2292, 4727, 6188, 7340, 24326, 34031, 41026, 66848, 104893, 106157, 125088, 230921, 295160

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OFFSET

1,2

COMMENTS

For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 9 followed by the digits 87 is prime (see Example section).

a(31) > 3*10^5.

LINKS

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

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

Makoto Kamada, Search for 79w87.

EXAMPLE

4 is in this sequence because 8*10^4 - 13 = 79987 is prime.

Initial terms and associated primes:

a(1) = 1, 67;

a(2) = 2, 787;

a(3) = 4, 79987;

a(4) = 7, 79999987;

a(5) = 8, 799999987; etc.

MATHEMATICA

Select[Range[1, 100000], PrimeQ[8*10^# - 13] &]

CROSSREFS

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

Sequence in context: A359727 A018614 A176745 * A212243 A336463 A358457

Adjacent sequences: A287205 A287206 A287207 * A287209 A287210 A287211

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, May 21 2017

EXTENSIONS

a(26)-a(28) from Robert Price, Aug 08 2019

a(29)-a(30) from Robert Price, Oct 26 2023

STATUS

approved