A089147 Numbers k such that 10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 3, 9, 13, 42, 51, 54, 91, 120, 168, 510, 819, 1071, 1756, 3010, 4333, 9534, 17839

Offset

1,2

Comments

Also numbers k such that (16*10^k - 7)/9 is prime.

a(19) > 10^5. - Robert Price, Dec 30 2014

Links

Table of n, a(n) for n=1..18.

Makoto Kamada, Factorizations of 177...77

Index entries for primes involving repunits

Mathematica

Do[ If[ PrimeQ[10^n + 7(10^n - 1)/9], Print[ n]], {n, 7000}]

Crossrefs

Cf. A002275, A088465 (corresponding primes).

Sequence in context: A305644 A316920 A018292 * A094043 A134190 A047905

Adjacent sequences: A089144 A089145 A089146 * A089148 A089149 A089150

Keyword

base,nonn

Author

Robert G. Wilson v, Dec 05 2003

Extensions

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

a(18) from Robert Price, Dec 30 2014

Status

approved