A056702 Numbers k such that 2*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 2, 3, 9, 15, 18, 36, 63, 114, 225, 405, 482, 1241, 2018, 9830, 15795, 22535, 27752, 41033, 95574, 131144, 156221

Offset: 1

Author

Robert G. Wilson v, Aug 10 2000

Keywords

• hard
• nonn

Comments

Also numbers k such that (25*10^k - 7)/9 is prime.

a(21) > 10^5. - Robert Price, Dec 17 2014

a(23) > 2*10^5. - Robert Price, May 19 2018

Links

• Makoto Kamada, Prime numbers of the form 277...77.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A093938.

Programs

Mathematica

Do[ If[ PrimeQ[ 2*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 6000}]

Extensions

a(15) from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(16)-a(20) from Robert Price, Dec 17 2014

a(21)-a(22) from Robert Price, May 19 2018