A271585 Numbers k such that (7*10^k + 143)/3 is prime.

1, 2, 3, 6, 7, 10, 11, 25, 26, 32, 122, 123, 126, 161, 292, 320, 743, 1630, 2738, 3178, 4814, 4833, 5030, 7035, 8151, 12554, 13954, 15113, 80490, 96112, 121487, 190683

Offset

1,2

Comments

For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 3 followed by the digits 81 is prime (see Example section).

a(33) > 2*10^5.

Links

Table of n, a(n) for n=1..32.

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

Makoto Kamada, Search for 23w81.

Example

3 is in this sequence because (7*10^3 + 143)/3 = 2381 is prime.
Initial terms and associated primes:
a(1) = 1, 71;
a(2) = 2, 281;
a(3) = 3, 2381;
a(4) = 6, 2333381;
a(5) = 7, 23333381, etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(7*10^# + 143)/3] &]

Prog

(PARI) is(n)=ispseudoprime((7*10^n+143)/3) \\ Charles R Greathouse IV, Jun 13 2017

Crossrefs

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

Sequence in context: A181498 A030703 A305927 * A351715 A285259 A284907

Adjacent sequences: A271582 A271583 A271584 * A271586 A271587 A271588

Keyword

nonn,more

Author

Robert Price, Apr 10 2016

Extensions

a(31)-a(32) from Robert Price, Mar 30 2018

Status

approved