A293029 Numbers k such that (7*10^k - 439)/9 is prime.

2, 20, 34, 35, 65, 73, 89, 94, 103, 157, 208, 217, 430, 508, 1220, 4729, 5218, 6350, 11810, 29893, 37753, 89539, 165350

Offset

1,1

Comments

For k > 1, numbers k such that k-2 occurrences of the digit 7 followed by the digits 29 is prime (see Example section).

a(24) > 2*10^5.

Links

Table of n, a(n) for n=1..23.

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

Makoto Kamada, Search for 7w29.

Example

2 is in this sequence because (7*10^2 - 439)/9 = 29 is prime.
Initial terms and associated primes:
a(1) = 2, 29;
a(2) = 20, 77777777777777777729;
a(3) = 34, 7777777777777777777777777777777729;
a(4) = 35, 77777777777777777777777777777777729;
a(5) = 65, 77777777777777777777777777777777777777777777777777777777777777729; etc.

Mathematica

Select[Range[1, 100000], PrimeQ[(4*10^# + 137)/3] &]

Crossrefs

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

Sequence in context: A112271 A201122 A103076 * A298761 A062133 A329729

Adjacent sequences: A293026 A293027 A293028 * A293030 A293031 A293032

Keyword

nonn,more,hard

Author

Robert Price, Sep 28 2017

Extensions

a(23) from Robert Price, Feb 14 2018

Status

approved