A294681 Numbers k such that (871*10^k - 7)/9 is prime.

1, 2, 7, 26, 28, 31, 38, 47, 59, 226, 233, 656, 752, 1073, 1808, 3646, 4252, 5246, 24686, 37952, 59599

Offset

1,2

Comments

Numbers such that the digits 96 followed by k occurrences of the digit 7 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 967w.

Example

2 is in this sequence because (871*10^2 - 7)/9 = 9677 is prime.
Initial terms and primes associated:
a(1) = 1, 967;
a(2) = 2, 9677;
a(3) = 7, 967777777;
a(4) = 26, 9677777777777777777777777777;
a(5) = 28, 967777777777777777777777777777; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(871*10^# - 7)/9] &]

Crossrefs

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

Sequence in context: A018980 A012990 A137102 * A229241 A335179 A091145

Adjacent sequences: A294678 A294679 A294680 * A294682 A294683 A294684

Keyword

nonn,more,hard

Author

Robert Price, Nov 06 2017

Status

approved