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 distinct solutions for n.

1, 1, 7, 21, 190, 1007, 6972, 111554, 1040635

Offset

1,3

Comments

The old entry with this sequence number was a duplicate of A048298.

Links

Table of n, a(n) for n=1..9.

Prog

(PARI) { npairs(n) = loca(r, q, z); r=0;
forvec(p=vector(n, i, [1, 2*n]),
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 }
/* Max Alekseyev, May 14 2008 */

Crossrefs

Sequence in context: A032032 A084711 A183938 * A357673 A111878 A133279

Adjacent sequences: A060143 A060144 A060145 * A060147 A060148 A060149

Keyword

nonn

Author

Jack Brennen, May 13 2008

Extensions

a(6)-a(8) from Max Alekseyev, May 14 2008

Edited by Charles R Greathouse IV, Oct 28 2009

a(9) from Sean A. Irvine, Oct 29 2022

Status

approved