A276047 Numbers k such that 4*10^k + 21 is prime.

1, 2, 3, 7, 15, 22, 30, 35, 44, 73, 89, 91, 224, 533, 821, 1037, 1338, 1458, 1777, 2046, 2257, 2877, 3047, 3407, 13398, 42766, 55906, 61625, 66653, 123113, 229836, 238163

Offset: 1

Author

Robert Price, Aug 17 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 21 is prime (see Example section).

a(33) > 3*10^5.

Examples

			3 is in this sequence because 4*10^3 + 21 = 4021 is prime.
Initial terms and associated primes:
a(1) = 1, 61;
a(2) = 2, 421;
a(3) = 3, 4021;
a(4) = 7, 40000021;
a(5) = 15, 4000000000000021, etc.
		

Links

• Makoto Kamada, Factorization of near-repdigit-related numbers.
• Makoto Kamada, Search for 40w21.

Crossrefs

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

Programs

Mathematica

Select[Range[0, 100000], PrimeQ[4*10^# + 21] &]

PARI

is(n)=ispseudoprime(4*10^n + 21) \\ Charles R Greathouse IV, Jun 13 2017

Extensions

a(30) from Robert Price, Dec 09 2018

a(31) - a(32) from Robert Price, Jun 01 2023