A287766 Numbers k such that (10^k*113 + 19)/3 is prime.

1, 9, 13, 33, 81, 1113, 1609, 5697, 13577, 49949

Offset: 1

Author

Mikk Heidemaa, May 31 2017

Keywords

• nonn
• hard
• more

Comments

For k > 2, numbers such that '37'||...'6'...||'73' ('6' concatenated k-2 times and prefixed by '37', and suffixed by '73') in decimal form is prime.

Examples

			13 is in the sequence because (10^13*113 + 19)/3 = 376666666666673 is prime.
		

Crossrefs

Cf. A051919, A101724.

Programs

Mathematica

ParallelTable[ If[ PrimeQ[ (10^n*113+19)/3], n, Nothing], {n, 2000}]

PARI

isok(n) = isprime((10^n*113 + 19)/3); \\ Michel Marcus, Jul 21 2017