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Admissibility in Finitely Generated Quasivarieties
Checking the admissibility of quasiequations in a finitely generated (i.e.,
generated by a finite set of finite algebras) quasivariety Q amounts to
checking validity in a suitable finite free algebra of the quasivariety, and is
therefore decidable. However, since free algebras may be large even for small
sets of small algebras and very few generators, this naive method for checking
admissibility in \Q is not computationally feasible. In this paper,
algorithms are introduced that generate a minimal (with respect to a multiset
well-ordering on their cardinalities) finite set of algebras such that the
validity of a quasiequation in this set corresponds to admissibility of the
quasiequation in Q. In particular, structural completeness (validity and
admissibility coincide) and almost structural completeness (validity and
admissibility coincide for quasiequations with unifiable premises) can be
checked. The algorithms are illustrated with a selection of well-known finitely
generated quasivarieties, and adapted to handle also admissibility of rules in
finite-valued logics
Automorphisms of polynomial algebras and Dirichlet series
Let GF(q)[x,y] be the polynomial algebra in two variables over the finite
field GF(q) with q elements. We give an exact formula and the asymptotics for
the number p(n) of automorphisms (f,g) of GF(q)[x,y] such that
max{deg(f),deg(g)}=n. We describe also the Dirichlet series generating function
p(1)/1^s+p(2)/2^s+p(3)/3^s+.... The same results hold for the automorphisms of
the free associative algebra GF(q). We have also obtained analogues for
free algebras with two generators in Nielsen - Schreier varieties of algebras.Comment: 13 pages, revised version, new results are adde
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