A089147 - OEIS

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

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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

MATHEMATICA

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

CROSSREFS

Cf. A002275, A088465 (corresponding primes).

KEYWORD

base,nonn

AUTHOR

EXTENSIONS

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

a(18) from Robert Price, Dec 30 2014

STATUS

approved