A281168 Numbers k such that (25*10^k + 71)/3 is prime.

1, 2, 4, 7, 10, 18, 22, 25, 38, 40, 154, 256, 282, 318, 640, 1095, 1126, 1214, 2588, 5616, 13159, 17644, 18631, 66714, 75465

Offset

1,2

Comments

For k>1, numbers such that the digit 8 followed by k-2 occurrences of the digit 3 followed by the digits 57 is prime (see Example section).

a(26) > 2*10^5.

Links

Table of n, a(n) for n=1..25.

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

Makoto Kamada, Search for 83w57.

Example

2 is in this sequence because (25*10^2 + 71) / 3 = 857 is prime.
Initial terms and primes associated:
a(1) = 1, 107;
a(2) = 2, 857;
a(3) = 4, 83357;
a(4) = 7, 83333357;
a(5) = 10, 83333333357; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(25*10^# + 71) / 3] &]

Crossrefs

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

Sequence in context: A049630 A032705 A332639 * A274352 A343328 A088762

Adjacent sequences: A281165 A281166 A281167 * A281169 A281170 A281171

Keyword

nonn,more,hard

Author

Robert Price, Jan 16 2017

Status

approved