

A320334


Number of ways to group the first 2*n positive integers into n pairs (xi, yi) with xi < yi, and such that the n numbers (yi  xi) mod n are all different.


1



1, 1, 0, 3, 12, 25, 0, 475, 5352, 17157, 0, 896535, 15083680, 68879713, 0
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OFFSET

0,4


COMMENTS



LINKS



EXAMPLE

a(3) = 3 because {(1,4), (2,6), (3,5)}, {(1,5), (2,4), (3,6)}, {(1,6), (2,5), (3,4)} are corresponding sets.


PROG

(C++) See Links section.


CROSSREFS



KEYWORD

nonn,more


AUTHOR



EXTENSIONS



STATUS

approved



