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

Author

Robert Price, Apr 10 2016

Keywords

• nonn
• more

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.

Examples

			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.
		

Links

• Makoto Kamada, Factorization of near-repdigit-related numbers.
• Makoto Kamada, Search for 23w81.

Crossrefs

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

Programs

Mathematica

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

PARI

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

Extensions

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