A293397 Numbers k such that 8*10^k + 87 is prime.

1, 2, 3, 6, 7, 8, 9, 21, 41, 51, 63, 67, 85, 174, 272, 297, 533, 697, 1326, 1344, 10668, 10814, 26252, 51721, 80342, 123657, 166227, 171296

Offset

1,2

Comments

For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 0 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 near-repdigit-related numbers.

Makoto Kamada, Search for 80w87.

Example

3 is in this sequence because 8*10^3 + 87 = 887 is prime.
Initial terms and associated primes:
a(1) = 1, 167;
a(2) = 2, 887;
a(3) = 3, 8087;
a(4) = 6, 8000087;
a(5) = 7, 80000087; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[8*10^# + 87] &]

Prog

(PARI) isok(k) = isprime(8*10^k + 87); \\ Altug Alkan, Oct 08 2017

Crossrefs

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

Sequence in context: A060132 A306556 A257262 * A059590 A276156 A144705

Adjacent sequences: A293394 A293395 A293396 * A293398 A293399 A293400

Keyword

nonn,more,hard

Author

Robert Price, Oct 08 2017

Extensions

a(26)-a(28) from Robert Price, Oct 19 2019

Status

approved