A256375 Numbers k such that 3*R_(k+2) - 10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

4, 5, 6, 8, 13, 14, 25, 30, 44, 58, 383, 455, 463, 504, 511, 1358, 2293, 4096, 4374, 8664, 13676, 23993, 84134, 90155, 115794, 127858

Offset: 1

Author

Robert Price, Apr 14 2015

Keywords

• more
• hard
• nonn

Comments

Also, numbers k such that (97*10^k - 1)/3 is prime.

Terms from Kamada.

a(27) > 250000.

Examples

			For k=4, 3*R_6 - 10^4 = 333333 - 10000 = 323333 which is prime.
		

Links

• Makoto Kamada, Near-repdigit numbers of the form ABAA...AA.
• Makoto Kamada, Prime numbers of the form 3233...33.
• Index entries for primes involving repunits.

Crossrefs

Cf. A002275.

Programs

Mathematica

Select[Range[0, 250000], PrimeQ[(97*10^#-1)/3 ] &]