A056678 - OEIS

This site is supported by donations to The OEIS Foundation.

A056678

Numbers n such that 20*R_n + 9 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

1, 2, 4, 13, 175, 415, 2504, 2758, 7924, 9400 (list; graph; refs; listen; history; internal format)

	OFFSET	`1,2`
	COMMENTS	`Also numbers n such that (2*10^(n+1)+61)/9 is prime.`
	LINKS	`Index entries for primes involving repunits` `Makoto Kamada, Factorizations of 22...229.`
	MATHEMATICA	`Do[ If[ PrimeQ[20*(10^n - 1)/9 + 9], Print[n]], {n, 5000}]`
	CROSSREFS	`Cf. A093401 (corresponding primes).` `Sequence in context: A103845 A055463 A091957 * A193232 A189486 A131758` `Adjacent sequences: A056675 A056676 A056677 * A056679 A056680 A056681`
	KEYWORD	`hard,nonn`
	AUTHOR	`Robert G. Wilson v (rgwv(AT)rgwv.com), Aug 10 2000`
	EXTENSIONS	`Two more terms from Rick L. Shepherd (rshepherd2(AT)hotmail.com), Mar 30 2004` `More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008`

Content is available under The OEIS End-User License Agreement .

Last modified February 17 03:20 EST 2012. Contains 205978 sequences.