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A056702 Numbers k such that 2*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k. 1
0, 2, 3, 9, 15, 18, 36, 63, 114, 225, 405, 482, 1241, 2018, 9830, 15795, 22535, 27752, 41033, 95574, 131144, 156221 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also numbers k such that (25*10^k - 7)/9 is prime.

a(21) > 10^5. - Robert Price, Dec 17 2014

a(23) > 2*10^5. - Robert Price, May 19 2018

LINKS

Table of n, a(n) for n=1..22.

Makoto Kamada, Prime numbers of the form 277...77.

Index entries for primes involving repunits

MATHEMATICA

Do[ If[ PrimeQ[ 2*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 6000}]

CROSSREFS

Cf. A002275, A093938.

Sequence in context: A078610 A108825 A109663 * A294126 A091361 A092352

Adjacent sequences:  A056699 A056700 A056701 * A056703 A056704 A056705

KEYWORD

hard,nonn

AUTHOR

Robert G. Wilson v, Aug 10 2000

EXTENSIONS

a(15) from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(16)-a(20) from Robert Price, Dec 17 2014

a(21)-a(22) from Robert Price, May 19 2018

STATUS

approved

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Last modified October 3 22:17 EDT 2022. Contains 357237 sequences. (Running on oeis4.)