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A056707 Numbers n such that 4*10^n + 3*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n. 2
1, 2, 16, 31, 37, 55, 62, 172, 174, 197, 727, 1246, 1752, 4318, 4328, 4930, 6450, 15206, 24041, 25321, 33068, 41484, 51511 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also numbers n such that (13*10^n-1)/3 is prime.

a(24) > 10^5. - Robert Price, Dec 21 2014

LINKS

Table of n, a(n) for n=1..23.

Makoto Kamada, Prime numbers of the form 433...33.

Index entries for primes involving repunits

EXAMPLE

43 and 433 are prime, so 1 and 2 are terms of the sequence.

MATHEMATICA

Do[ If[ PrimeQ[ 4*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 5000}]

CROSSREFS

Cf. A002275, A093673.

Sequence in context: A242933 A321308 A109210 * A069256 A279034 A120069

Adjacent sequences:  A056704 A056705 A056706 * A056708 A056709 A056710

KEYWORD

hard,nonn

AUTHOR

Robert G. Wilson v, Aug 10 2000

EXTENSIONS

26044 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(18)-a(23) from Robert Price, Dec 21 2014

STATUS

approved

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Last modified June 12 14:45 EDT 2021. Contains 344957 sequences. (Running on oeis4.)