A294917 Numbers k such that 4*10^k - 71 is prime.

3, 4, 6, 11, 12, 17, 21, 54, 82, 112, 189, 235, 426, 442, 4897, 12463, 14383, 18630, 38669, 95752, 108238, 179265

Offset

1,1

Comments

For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 9 followed by the digits 29 is prime (see Example section).

a(23) > 2*10^5.

Links

Table of n, a(n) for n=1..22.

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

Makoto Kamada, Search for 39w29.

Example

3 is in this sequence because 4*10^3 - 71 = 3929 is prime.
Initial terms and associated primes:
a(1) = 3, 3929;
a(2) = 4, 39929;
a(3) = 6, 3999929;
a(4) = 11, 399999999929;
a(5) = 12, 3999999999929; etc.

Mathematica

Select[Range[2, 100000], PrimeQ[4*10^# - 71] &]

Crossrefs

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

Sequence in context: A275418 A047413 A331191 * A115018 A057030 A230430

Adjacent sequences: A294914 A294915 A294916 * A294918 A294919 A294920

Keyword

nonn,more,hard

Author

Robert Price, Nov 10 2017

Extensions

a(21)-a(22) from Robert Price, Dec 28 2018

Status

approved