1,3

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

(PARI) /* from Max Alekseyev */

{ 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 }

Sequence in context: A028248 A032032 A084711 this_sequence A111878 A133279 A082826

Adjacent sequences: A060143 A060144 A060145 this_sequence A060147 A060148 A060149

nonn

Jack Brennen (jb(AT)brennen.net), May 13 2008

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