

A076017


Number of nonisomorphic systems with n elements having with one binary operation satisfying the equation B(AB)=A (semisymmetric quasigroups).


1



1, 1, 2, 3, 4, 9, 41, 595, 26620, 3908953, 1867918845
(list;
graph;
refs;
listen;
history;
text;
internal format)



OFFSET

1,3


COMMENTS

In January of 1968, Don Knuth described the concept of what he called an "abstract grope" to the students in his class for sophomore math majors at Caltech.
The students had just learned about abstract groups and he wanted them to get experience doing research with other algebraic axioms; so he challenged them to prove as many interesting things as they could about sets of elements with a binary operator that satisfies the identity x(yx)=y.
The name came from the fact that they were groping for results. Such systems later acquired a more complicated and more dignified yet less memorable name, "semisymmetric quasigroups". The students came up with some good stuff, including the concept of normal subgropes.


REFERENCES

D. E. Knuth, The Art of Computer Programming, Vol. 4B, in preparation.


LINKS

Table of n, a(n) for n=1..11.


CROSSREFS

Cf. A076016A076021.
Sequence in context: A086432 A186928 A076018 * A245366 A135112 A286709
Adjacent sequences: A076014 A076015 A076016 * A076018 A076019 A076020


KEYWORD

nonn,hard,more


AUTHOR

Richard C. Schroeppel, Oct 29 2002


EXTENSIONS

a(10), a(11) and comments from Don Knuth, May 12 2005  May 14 2005


STATUS

approved



