A282256 Numbers k such that (142*10^k - 7)/9 is prime.

1, 8, 16, 23, 32, 86, 202, 214, 332, 443, 760, 1066, 1270, 2210, 4189, 5759, 6886, 9286, 21205, 22139, 24784, 35021, 63734, 112352

Offset

1,2

Comments

Numbers such that the digits 15 followed by k occurrences of the digit 7 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 157w.

Example

8 is in this sequence because (142*10^8 - 7)/9 = 1577777777 is prime.
Initial terms and primes associated:
a(1) = 1, 157;
a(2) = 8, 1577777777;
a(3) = 16, 157777777777777777;
a(4) = 23, 1577777777777777777777777;
a(5) = 32, 1577777777777777777777777777777777; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(142*10^# - 7)/9] &]

Crossrefs

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

Sequence in context: A096912 A254141 A188438 * A037370 A325261 A333195

Adjacent sequences: A282253 A282254 A282255 * A282257 A282258 A282259

Keyword

nonn,more,hard

Author

Robert Price, Feb 12 2017

Extensions

a(24) from Robert Price, Dec 31 2019

Status

approved