A060146 - OEIS

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A060146

Take the first 2n integers and using each integer once and only once as either a numerator or a denominator, construct n fractions whose sum is an integer; a(n) = number of unique solutions for n.

1, 1, 7, 21, 190, 1007, 6972, 111554 (list; graph; refs; listen; history; internal format)

	OFFSET	`1,3`
	COMMENTS	`The old entry with this sequence number was a duplicate of A048298.`
	PROG	`(PARI) /* from Max Alekseyev /` `{ npairs(n) = loca(r, q, z); r=0;` `forvec(p=vector(n, i, [1, 2n]),` `q = eval( setminus( Set(vector(2*n, i, i)), Set(p) ) );` `for(j=1, n!,` `z=numtoperm(n, j);` `if(type( sum(j=1, #p, p[j]/q[z[j]]) )=="t_INT", r++); ); , 2); r }`
	CROSSREFS	`Sequence in context: A032032 A084711 A183938 * A111878 A133279 A192734` `Adjacent sequences: A060143 A060144 A060145 * A060147 A060148 A060149`
	KEYWORD	`nonn`
	AUTHOR	`Jack Brennen (jb(AT)brennen.net), May 13 2008`
	EXTENSIONS	`a(6)-a(8) from Max Alekseyev (maxale(AT)gmail.com), May 14 2008` `Edited by Charles R Greathouse IV (charles.greathouse(AT)case.edu), Oct 28 2009`

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Last modified February 16 05:55 EST 2012. Contains 205860 sequences.