A280969 Numbers k such that 7*10^k - 17 is prime.

1, 2, 3, 11, 16, 33, 35, 37, 39, 42, 64, 94, 123, 136, 308, 309, 310, 357, 965, 1109, 2178, 8916, 9471, 18330, 62127, 252209

Offset

1,2

Comments

For k>1, numbers such that the digit 6 followed by k-2 occurrences of the digit 9 followed by the digits 83 is prime (see Example section).

a(27) > 3*10^5.

Links

Table of n, a(n) for n=1..26.

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

Makoto Kamada, Search for 69w83.

Example

3 is in this sequence because 7*10^3- 17 = 6983 is prime.
Initial terms and primes associated:
a(1) = 1, 53;
a(2) = 2, 683;
a(3) = 3, 6983;
a(4) = 11, 699999999983;
a(5) = 16, 69999999999999983; etc.

Mathematica

Select[Range[1, 100000], PrimeQ[7*10^# - 17] &]

Crossrefs

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

Sequence in context: A354742 A228520 A361127 * A349565 A091734 A341784

Adjacent sequences: A280966 A280967 A280968 * A280970 A280971 A280972

Keyword

nonn,more,hard

Author

Robert Price, Jan 11 2017

Extensions

Comment corrected by Harvey P. Dale, Apr 10 2019

a(26) from Robert Price, May 31 2023

Status

approved