A056702 - OEIS

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A056702

Numbers n such that 2*10^n + 7*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n.

0, 2, 3, 9, 15, 18, 36, 63, 114, 225, 405, 482, 1241, 2018, 9830 (list; graph; refs; listen; history; internal format)

	OFFSET	`1,2`
	COMMENTS	`Also numbers n such that (25*10^n-7)/9 is prime.`
	LINKS	`Makoto Kamada, Factorizations of 277...77` `Index entries for primes involving repunits`
	MATHEMATICA	`Do[ If[ PrimeQ[ 210^n + 7(10^n-1)/9], Print[n]], {n, 0, 6000}]`
	CROSSREFS	`Sequence in context: A078610 A108825 A109663 * A091361 A092352 A061933` `Adjacent sequences: A056699 A056700 A056701 * A056703 A056704 A056705`
	KEYWORD	`hard,nonn`
	AUTHOR	`Robert G. Wilson v (rgwv(AT)rgwv.com), Aug 10 2000`
	EXTENSIONS	`9830 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008`

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Last modified February 16 19:23 EST 2012. Contains 205945 sequences.