1,367 research outputs found
Belyi-extending maps and the Galois action on dessins d'enfants
We study the absolute Galois group by looking for invariants and orbits of
its faithful action on Grothendieck's dessins d'enfants. We define a class of
functions called Belyi-extending maps, which we use to construct new Galois
invariants of dessins from previously known invariants. Belyi-extending maps
are the source of the ``new-type'' relations on the injection of the absolute
Galois group into the Grothendieck-Teichmuller group. We make explicit how to
get from a general Belyi-extending map to formula for its associated invariant
which can be implemented in a computer algebra package. We give an example of a
new invariant differing on two dessins which have the same values for the other
readily computable invariants.Comment: 13 pages, 7 figures; submitted for publication; revisions are that
the paper now deals only with Galois invariants of dessins, and that material
is slightly expande
Parametrizing quartic algebras over an arbitrary base
We parametrize quartic commutative algebras over any base ring or scheme
(equivalently finite, flat degree four -schemes), with their cubic
resolvents, by pairs of ternary quadratic forms over the base. This generalizes
Bhargava's parametrization of quartic rings with their cubic resolvent rings
over by pairs of integral ternary quadratic forms, as well as
Casnati and Ekedahl's construction of Gorenstein quartic covers by certain rank
2 families of ternary quadratic forms. We give a geometric construction of a
quartic algebra from any pair of ternary quadratic forms, and prove this
construction commutes with base change and also agrees with Bhargava's explicit
construction over .Comment: submitte
Rings and ideals parametrized by binary n-ic forms
The association of algebraic objects to forms has had many important
applications in number theory. Gauss, over two centuries ago, studied quadratic
rings and ideals associated to binary quadratic forms, and found that ideal
classes of quadratic rings are exactly parametrized by equivalence classes of
integral binary quadratic forms. Delone and Faddeev, in 1940, showed that cubic
rings are parametrized by equivalence classes of integral binary cubic forms.
Birch, Merriman, Nakagawa, Corso, Dvornicich, and Simon have all studied rings
associated to binary forms of degree n for any n, but it has not previously
been known which rings, and with what additional structure, are associated to
binary forms. In this paper, we show exactly what algebraic structures are
parametrized by binary n-ic forms, for all n. The algebraic data associated to
an integral binary n-ic form includes a ring isomorphic to as a
-module, an ideal class for that ring, and a condition on the ring
and ideal class that comes naturally from geometry. In fact, we prove these
parametrizations when any base scheme replaces the integers, and show that the
correspondences between forms and the algebraic data are functorial in the base
scheme. We give geometric constructions of the rings and ideals from the forms
that parametrize them and a simple construction of the form from an appropriate
ring and ideal.Comment: submitte
- …