A267865 Numbers k such that 49*10^k + 1 is prime.

1, 4, 5, 7, 9, 10, 18, 19, 34, 52, 71, 180, 238, 331, 370, 576, 925, 994, 1075, 1841, 2460, 2857, 6007, 6193, 10836, 18732, 58708, 72861

Offset: 1

Author

Robert Price, Apr 07 2016

Keywords

• nonn
• more

Comments

Numbers k such that the digits 49 followed by k-1 occurrences of the digit 0 followed by the digit 1 is prime (see Example section).

a(29) > 10^5.

Examples

			4 is in this sequence because 49*10^4 + 1 = 490001 is prime.
Initial terms and associated primes:
a(1) = 1, 491;
a(2) = 4, 490001;
a(3) = 5, 4900001;
a(4) = 7, 490000001;
a(5) = 9, 49000000001, etc.
		

Links

• Makoto Kamada, Factorization of near-repdigit-related numbers.
• Makoto Kamada, Search for 490w1.

Crossrefs

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

Programs

Mathematica

Select[Range[0, 100000], PrimeQ[49*10^#+1] &]

PARI

is(n)=ispseudoprime(49*10^n+1) \\ Charles R Greathouse IV, Jun 13 2017