id:A109906 - OEIS Search Results

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a(n) = number of primes of the form k*(n-k) +1.

+0
6

0, 1, 1, 1, 2, 0, 3, 2, 1, 1, 4, 1, 6, 1, 1, 2, 8, 1, 5, 3, 1, 4, 7, 1, 7, 1, 4, 5, 8, 0, 10, 6, 2, 2, 7, 1, 9, 8, 4, 4, 14, 1, 16, 3, 3, 5, 12, 3, 7, 7, 4, 11, 21, 0, 11, 4, 7, 6, 11, 2, 12, 9, 7, 10, 7, 1, 22, 7, 7, 5, 17, 3, 23, 10, 2, 9, 19, 2, 19, 8, 5, 8, 23, 1, 16, 6, 4, 11, 14, 4, 16, 12, 9, 5, 12 (list; graph; listen)

	OFFSET	`2,5`
	COMMENT	`7 is the only number which gives primes for all possible values of k.`
	EXAMPLE	`a(7) = 3, 16 +1 =7, 25 +1 =11, 3*4 +1 = 13.`
	PROGRAM	`(PARI) { a(n)=local(r); r=0; for(k=1, n\2, if(isprime(k*(n-k)+1), r++)); r } (Alekseyev)`
	CROSSREFS	`Cf. A109904, A109905.` `Sequence in context: A115241 A097418 A026728 this_sequence A090722 A082785 A100949` `Adjacent sequences: A109903 A109904 A109905 this_sequence A109907 A109908 A109909`
	KEYWORD	`nonn`
	AUTHOR	`Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Jul 15 2005`
	EXTENSIONS	`More terms from Max Alekseyev (maxal(AT)cs.ucsd.edu), Oct 04 2005`

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Last modified July 26 23:19 EDT 2008. Contains 142293 sequences.

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