A056660 - OEIS

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A056660

Numbers n such that 20*R_n + 1 is a prime, where R_n = 11...1 is the repunit (A002275) of length n.

3, 17, 99, 120, 243, 545, 630, 1493, 2565, 8087 (list; graph; refs; listen; history; internal format)

	OFFSET	`1,1`
	COMMENTS	`Also numbers n such that (2*10^(n+1)-11)/9 is a prime.`
	LINKS	`Index entries for primes involving repunits` `Makoto Kamada, Factorizations of 22...221.`
	MATHEMATICA	`Do[ If[ PrimeQ[ 20*(10^n - 1)/9 + 1 ], Print[n]], {n, 7000}]`
	CROSSREFS	`Cf. A091189 (corresponding primes).` `Sequence in context: A151330 A010913 A142988 * A155610 A001541 A161473` `Adjacent sequences: A056657 A056658 A056659 * A056661 A056662 A056663`
	KEYWORD	`nonn`
	AUTHOR	`Robert G. Wilson v (rgwv(AT)rgwv.com), Aug 09 2000`
	EXTENSIONS	`a(8) and a(9) from Rick L. Shepherd (rshepherd2(AT)hotmail.com), Feb 22 2004` `More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008`

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Last modified February 17 23:08 EST 2012. Contains 206085 sequences.