A056682 - OEIS

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A056682

Numbers n such that 40*R_n + 7 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

0, 1, 3, 9, 19, 25, 721, 1309, 3169 (list; graph; refs; listen; history; internal format)

	OFFSET	`1,3`
	COMMENTS	`Also numbers n such that (4*10^(n+1)+23)/9 is prime.`
	LINKS	`Index entries for primes involving repunits`
	MATHEMATICA	`Do[ If[ PrimeQ[40*(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}]`
	CROSSREFS	`Sequence in context: A084670 A002091 A056259 * A032670 A068677 A069832` `Adjacent sequences: A056679 A056680 A056681 * A056683 A056684 A056685`
	KEYWORD	`hard,nonn`
	AUTHOR	`Robert G. Wilson v (rgwv(AT)rgwv.com), Aug 10 2000`

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Last modified February 15 15:20 EST 2012. Contains 205823 sequences.