A294131 Numbers k such that 5*10^k - 13 is prime.

1, 2, 3, 15, 20, 30, 90, 120, 125, 141, 326, 464, 491, 494, 540, 558, 822, 1103, 1463, 1638, 1898, 3332, 10671, 24842, 37412, 48855, 173904

Offset

1,2

Comments

For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 9 followed by the digits 87 is prime (see Example section).

a(28) > 2*10^5.

Links

Table of n, a(n) for n=1..27.

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

Makoto Kamada, Search for 49w87.

Example

2 is in this sequence because 5*10^2 - 13 = 487 is prime.
Initial terms and associated primes:
a(1) = 1, 37;
a(2) = 2, 487;
a(3) = 3, 4987;
a(4) = 15, 4999999999999987;
a(5) = 20, 499999999999999999987; etc.

Maple

select(k->isprime(5*10^k-13), [$1..1000]); # Muniru A Asiru, Jan 03 2019

Mathematica

Select[Range[1, 100000], PrimeQ[5*10^# - 13] &]

Crossrefs

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

Sequence in context: A299486 A173334 A331089 * A274003 A101507 A047176

Adjacent sequences: A294128 A294129 A294130 * A294132 A294133 A294134

Keyword

nonn,more,hard

Author

Robert Price, Oct 23 2017

Extensions

a(27) from Robert Price, Jan 03 2019

Status

approved