A281641 Numbers k such that (5*10^k - 107)/3 is prime.

2, 4, 5, 7, 8, 10, 46, 60, 204, 324, 348, 1268, 1525, 2152, 2250, 3398, 4073, 4939, 6835, 12502, 26012, 28339, 50861, 53855, 84469, 86011

Offset

1,1

Comments

For k>1, numbers such that the digit 1 followed by k-2 occurrences of the digit 6 followed by the digits 31 is prime (see Example section).

a(27) > 2*10^5.

Links

Table of n, a(n) for n=1..26.

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

Makoto Kamada, Search for 16w31.

Example

4 is in this sequence because (5*10^4 - 107)/3 = 16631 is prime.
Initial terms and primes associated:
a(1) = 2, 131;
a(2) = 4, 16631;
a(3) = 5, 166631;
a(4) = 7, 16666631;
a(5) = 8, 166666631; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(5*10^# - 107)/3] &]

Crossrefs

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

Sequence in context: A364238 A085370 A006693 * A158850 A274427 A187576

Adjacent sequences: A281638 A281639 A281640 * A281642 A281643 A281644

Keyword

nonn,more,hard

Author

Robert Price, Jan 25 2017

Status

approved