A273097 Numbers k such that 4*10^k + 87 is prime.

1, 2, 4, 5, 13, 25, 27, 32, 37, 38, 40, 45, 57, 80, 91, 151, 214, 441, 644, 764, 797, 1222, 2329, 2931, 4324, 21794, 22396, 24041, 46420, 51489, 55165, 126625

Offset: 1

Author

Robert Price, May 15 2016

Keywords

• nonn
• more

Comments

For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 0 followed by the digits 87 is prime (see Example section).

a(33) > 3*10^5.

Examples

			4 is in this sequence because 4*10^4 + 87 = 40087 is prime.
Initial terms and associated primes:
a(1) = 1, 127:
a(2) = 2, 487;
a(3) = 4, 40087;
a(4) = 5, 400087;
a(5) = 13, 40000000000087, etc.
		

Links

• Makoto Kamada, Factorization of near-repdigit-related numbers.
• Makoto Kamada, Search for 40w87.

Crossrefs

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

Programs

Mathematica

Select[Range[0, 100000], PrimeQ[4*10^# + 87] &]

PARI

is(n)=ispseudoprime(4*10^n + 87) \\ Charles R Greathouse IV, Jun 13 2017

Extensions

a(32) from Robert Price, Aug 15 2018