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

1,2

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

Table of n, a(n) for n=1..22.

Makoto Kamada, Prime numbers of the form 277...77.

Index entries for primes involving repunits

Mathematica

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

Crossrefs

Cf. A002275, A093938.

Sequence in context: A078610 A108825 A109663 * A294126 A091361 A092352

Adjacent sequences: A056699 A056700 A056701 * A056703 A056704 A056705

Keyword

hard,nonn

Author

Robert G. Wilson v, Aug 10 2000

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

Status

approved