A281110 Numbers k such that (25*10^k + 161)/3 is prime.

1, 2, 3, 6, 12, 14, 29, 31, 35, 58, 77, 79, 108, 122, 154, 895, 1045, 1763, 3243, 3605, 9446, 16119, 19238, 28562, 87726, 89413, 107804, 136303

Offset

1,2

Comments

For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 3 followed by the digits 87 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 83w87.

Example

3 is in this sequence because (25*10^3 + 161) / 3 = 8387 is prime.
Initial terms and associated primes:
a(1) = 1, 137;
a(2) = 2, 887;
a(3) = 3, 8387;
a(4) = 6, 8333387;
a(5) = 12, 8333333333387; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(25*10^# + 161) / 3] &]

Crossrefs

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

Sequence in context: A110590 A329401 A291174 * A252792 A284638 A111271

Adjacent sequences: A281107 A281108 A281109 * A281111 A281112 A281113

Keyword

nonn,more,hard

Author

Robert Price, Jan 14 2017

Extensions

a(27)-a(28) from Robert Price, Sep 30 2019

Status

approved