

A287208


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


0



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
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OFFSET

1,2


COMMENTS

For k>1, numbers such that the digit 7 followed by k2 occurrences of the digit 9 followed by the digits 87 is prime (see Example section).
a(29) > 2*10^5.


LINKS

Table of n, a(n) for n=1..28.
Makoto Kamada, Factorization of nearrepdigitrelated numbers.
Makoto Kamada, Search for 79w87.


EXAMPLE

4 is in this sequence because 8*10^4  13 = 79987 is prime.
Initial terms and primes associated:
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: A190808 A018614 A176745 * A212243 A336463 A306235
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


STATUS

approved



