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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
1, 3, 9, 13, 42, 51, 54, 91, 120, 168, 510, 819, 1071, 1756, 3010, 4333, 9534, 17839 (list; graph; refs; listen; history; text; internal format)
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).
Sequence in context: A305644 A316920 A018292 * A094043 A134190 A047905
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

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Last modified May 18 09:54 EDT 2024. Contains 372620 sequences. (Running on oeis4.)