A281513 Numbers k such that 5*10^k - 43 is prime.

1, 2, 3, 4, 5, 6, 10, 22, 23, 30, 51, 65, 110, 124, 248, 1254, 1416, 1515, 1797, 6262, 9264, 12213, 17382, 17884, 19502, 22408, 116966, 192422

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 57 is prime (see Example section).

a(29) > 2*10^5.

Links

Table of n, a(n) for n=1..28.

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

Makoto Kamada, Search for 49w57.

Example

3 is in this sequence because 5*10^3 - 43 = 4957 is prime.
Initial terms and associated primes:
a(1) = 1, 7;
a(2) = 2, 457;
a(3) = 3, 4957;
a(4) = 4, 49957;
a(5) = 5, 499957; etc.

Mathematica

Select[Range[1, 100000], PrimeQ[5*10^# - 43] &]

Crossrefs

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

Sequence in context: A337880 A271341 A306109 * A270890 A010350 A306581

Adjacent sequences: A281510 A281511 A281512 * A281514 A281515 A281516

Keyword

nonn,more,hard

Author

Robert Price, Apr 13 2017

Extensions

a(27)-a(28) from Robert Price, Jan 24 2019

Status

approved