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A056708 Numbers k such that 4*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k. 1
1, 4, 13, 25, 36, 357, 373, 1041, 1089, 1093, 1297, 8274, 10732, 15972, 18114, 21823, 34519, 36096, 75498 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
Also numbers k such that (43*10^k - 7)/9 is prime.
a(20) > 10^5. - Robert Price, Dec 22 2014
LINKS
MATHEMATICA
Do[ If[ PrimeQ[ 4*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 6000}]
CROSSREFS
Sequence in context: A272734 A273414 A154820 * A307271 A333297 A092484
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Aug 10 2000
EXTENSIONS
a(12) from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(13)-a(19) from Robert Price, Dec 22 2014
STATUS
approved

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