A294680 Numbers k such that (85*10^k + 293)/9 is prime.

1, 2, 4, 10, 23, 41, 47, 61, 137, 278, 388, 523, 992, 3716, 4438, 5185, 7553, 9970, 17962, 18568, 36796, 112919, 114737, 116842

Offset

1,2

Comments

For k>1, numbers such that the digit 9 followed by k-2 occurrences of the digit 4 followed by the digits 77 is prime (see Example section).

a(25) > 2*10^5.

Links

Table of n, a(n) for n=1..24.

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

Makoto Kamada, Search for 94w77.

Example

2 is in this sequence because (85*10^2 + 293)/9 = 977 is prime.
Initial terms and primes associated:
a(1) = 1, 127;
a(2) = 2, 977;
a(3) = 4, 94477;
a(4) = 10, 94444444477;
a(5) = 23, 944444444444444444444477; etc.

Mathematica

Select[Range[1, 100000], PrimeQ[(85*10^# + 293)/9] &]

Crossrefs

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

Sequence in context: A179490 A337520 A173185 * A189890 A189587 A345195

Adjacent sequences: A294677 A294678 A294679 * A294681 A294682 A294683

Keyword

nonn,more,hard

Author

Robert Price, Nov 06 2017

Extensions

a(22)-a(24) from Robert Price, Dec 13 2019

Status

approved