id:A072341 - OEIS Search Results

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a(n) = the least natural number k such that k*sigma(n) + 1 is prime.

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1, 2, 1, 4, 1, 1, 2, 2, 4, 1, 1, 1, 2, 3, 3, 10, 1, 2, 2, 1, 3, 1, 3, 1, 10, 1, 1, 2, 1, 1, 3, 2, 2, 2, 2, 6, 5, 1, 2, 2, 1, 1, 2, 4, 1, 1, 2, 3, 4, 4, 1, 2, 2, 2, 1, 2, 3, 2, 1, 2, 5, 1, 3, 4, 4, 3, 2, 1, 1, 3, 1, 6, 2, 2, 3, 2, 1, 2, 3, 2, 6, 1, 4, 2, 1, 3, 2, 1, 2, 4, 1, 2, 2, 3, 2, 3, 2, 12, 1, 6, 1, 2, 3 (list; graph; listen)

	OFFSET	`1,2`
	COMMENT	`Conjecture: a(n) is less than or equal to n for all n.`
	EXAMPLE	`sigma(4) = 7, and the least natural number k such that 7 k + 1 is prime is k = 4; so a(4) = 4.`
	MATHEMATICA	`f[n_] := Module[{i}, i = 0; While[ ! PrimeQ[i*DivisorSigma[1, n] + 1], i++ ]; i]; Table[f[i], {i, 1, 150}]`
	CROSSREFS	`Cf. A034694.` `Adjacent sequences: A072338 A072339 A072340 this_sequence A072342 A072343 A072344` `Sequence in context: A131642 A070674 A070668 this_sequence A011130 A072918 A140582`
	KEYWORD	`nonn`
	AUTHOR	`Joseph L. Pe (joseph_l_pe(AT)hotmail.com), Jul 16 2002`

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Last modified October 7 14:39 EDT 2008. Contains 144666 sequences.

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