id:A066728 - OEIS Search Results

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a(n) is the number of integers of the form (n+k+n*k)/(n-k) for k= 1,2,...,n-1.

+0
2

0, 1, 1, 3, 1, 4, 2, 4, 2, 7, 1, 7, 3, 5, 3, 8, 1, 11, 3, 7, 3, 9, 2, 9, 5, 7, 3, 15, 1, 13, 3, 6, 7, 11, 3, 11, 3, 9, 3, 19, 1, 15, 5, 7, 5, 11, 2, 17, 5, 11, 3, 15, 3, 19, 7, 9, 3, 15, 1, 15, 5, 7, 11, 15, 3, 15, 3, 15, 3, 29, 1, 14, 5, 7, 11, 15, 3, 23, 4, 11, 4, 15, 3, 15, 7, 9, 3, 29, 3, 23 (list; graph; listen)

	OFFSET	`1,4`
	COMMENT	`a(n)=1 iff n is 2 or the lesser of twin primes (for n>=3, n follows the sequence A001359).` `Also the number of factors of n*(n+2) which are less than n. - Antonio G. Astudillo (afg_astudillo(AT)lycos.com), Jul 02 2003`
	FORMULA	`a(n)=ceil( d(n*(n+2)) / 2 ) - 1, where d(n) = number of divisors of n (A000005). - Antonio G. Astudillo (afg_astudillo(AT)lycos.com), Jul 02 2003`
	EXAMPLE	`Example: (4+1+41)/(4-1); (4+2+42)/(4-2); (4+3+4*3)/(4-1) are integers hence a(4)=3.`
	MAPLE	`with(numtheory):A066728 := n->ceil(tau(n*(n+2))/2)-1;`
	CROSSREFS	`Cf. A063091.` `Sequence in context: A116537 A048225 A075148 this_sequence A066899 A139432 A115208` `Adjacent sequences: A066725 A066726 A066727 this_sequence A066729 A066730 A066731`
	KEYWORD	`nonn`
	AUTHOR	`Benoit Cloitre (benoit7848c(AT)orange.fr), Jan 15 2002`

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Last modified September 5 19:27 EDT 2008. Contains 143485 sequences.

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