A056662 Numbers k such that 90*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 2, 16, 139, 989, 1886, 3529, 5995, 13819, 21872, 26044, 87719, 232598

Offset: 1

Author

Robert G. Wilson v, Aug 09 2000

Keywords

• hard
• nonn

Comments

Also numbers k such that 10^(k+1) - 3 is a prime number.

Links

• Makoto Kamada, Prime numbers of the form 99...997.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A089675.

Programs

Mathematica

Do[ If[ PrimeQ[ 90*(10^n - 1)/9 + 7 ], Print[n]], {n, 0, 1250}]

Formula

a(n) = A089675(n) - 1. - Robert Price, Nov 01 2014

Extensions

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(13)-a(14) derived from A089675 by Robert Price, Nov 01 2014