A294914 Numbers k such that (14*10^k - 131)/9 is prime.

4, 7, 10, 27, 40, 43, 55, 61, 63, 117, 259, 417, 427, 561, 568, 889, 1335, 26625, 36613, 45261, 47205

Offset

1,1

Comments

For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 5 followed by the digits 47 is prime (see Example section).

a(22) > 2*10^5.

Links

Table of n, a(n) for n=1..21.

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

Makoto Kamada, Search for 15w41.

Example

4 is in this sequence because (14*10^4 - 131)/9 = 15541 is prime.
Initial terms and associated primes:
a(1) = 4, 15541;
a(2) = 7, 15555541;
a(3) = 10, 15555555541;
a(4) = 27, 1555555555555555555555555541;
a(5) = 40, 15555555555555555555555555555555555555541; etc.

Mathematica

Select[Range[1, 100000], PrimeQ[(14*10^# - 131)/9] &]

Crossrefs

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

Sequence in context: A182278 A032818 A123986 * A371458 A088405 A100591

Adjacent sequences: A294911 A294912 A294913 * A294915 A294916 A294917

Keyword

nonn,more,hard

Author

Robert Price, Nov 10 2017

Status

approved