A276118 Numbers k such that 42 * 10^k + 1 is prime.

0, 1, 2, 4, 13, 19, 39, 62, 76, 79, 109, 184, 222, 265, 370, 626, 670, 679, 763, 1950, 2174, 3379, 7369, 9087, 34990, 47535, 97970

Offset: 1

Author

Robert Price, Aug 20 2016

Keywords

• nonn
• more

Comments

For k > 0, numbers k such that the digits 42 followed by k - 1 occurrences of the digit 0 followed by the digit 1 is prime (see the Example section).

a(28) > 10^5.

Examples

			4 is in this sequence because 42*10^4+1 = 420001 is prime.
Initial terms and associated primes:
a(1) = 0, 43;
a(2) = 1, 421;
a(3) = 2, 4201;
a(4) = 4, 420001;
a(5) = 13, 420000000000001, etc.
		

Links

• Makoto Kamada, Factorization of near-repdigit-related numbers.
• Makoto Kamada, Search for 420w1.

Crossrefs

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

Programs

Mathematica

Select[Range[0, 100000], PrimeQ[42 * 10^# + 1] &]

PARI

is(n)=ispseudoprime(42*10^n+1) \\ Charles R Greathouse IV, Jun 13 2017