585 research outputs found
On the simplest sextic fields and related Thue equations
We consider the parametric family of sextic Thue equations where
is an integer and is a divisor of . We
show that the only solutions to the equations are the trivial ones with
.Comment: 12 pages, 2 table
Birational classification of fields of invariants for groups of order
Let be a finite group acting on the rational function field
by -automorphisms for
any . Noether's problem asks whether the invariant field
is rational (i.e. purely transcendental) over
. Saltman and Bogomolov, respectively, showed that for any prime
there exist groups of order and of order such that
is not rational over by showing the non-vanishing
of the unramified Brauer group: . For , Chu,
Hu, Kang and Prokhorov proved that if is a 2-group of order , then
is rational over . Chu, Hu, Kang and Kunyavskii
showed that if is of order 64, then is rational over
except for the groups belonging to the two isoclinism families
and . Bogomolov and B\"ohning's theorem claims that if
and belong to the same isoclinism family, then
and are stably -isomorphic. We investigate the
birational classification of for groups of order 128 with
. Moravec showed that there exist exactly 220
groups of order 128 with forming 11
isoclinism families . We show that if and belong to
or (resp. or ), then and
are stably -isomorphic with
. Explicit structures of non-rational
fields are given for each cases including also the case
with .Comment: 31 page
Rationality problem for algebraic tori
We give the complete stably rational classification of algebraic tori of
dimensions and over a field . In particular, the stably rational
classification of norm one tori whose Chevalley modules are of rank and
is given. We show that there exist exactly (resp. , resp. )
stably rational (resp. not stably but retract rational, resp. not retract
rational) algebraic tori of dimension , and there exist exactly
(resp. , resp. ) stably rational (resp. not stably but retract
rational, resp. not retract rational) algebraic tori of dimension . We make
a procedure to compute a flabby resolution of a -lattice effectively by
using the computer algebra system GAP. Some algorithms may determine whether
the flabby class of a -lattice is invertible (resp. zero) or not. Using the
algorithms, we determine all the flabby and coflabby -lattices of rank up to
and verify that they are stably permutation. We also show that the
Krull-Schmidt theorem for -lattices holds when the rank , and fails
when the rank is . Indeed, there exist exactly (resp. )
-lattices of rank (resp. ) which are decomposable into two different
ranks. Moreover, when the rank is , there exist exactly -lattices
which are decomposable into the same ranks but the direct summands are not
isomorphic. We confirm that for any Bravais group of dimension
where is the flabby class of the corresponding -lattice of
rank . In particular, for any maximal finite subgroup where . As an application of the methods
developed, some examples of not retract (stably) rational fields over are
given.Comment: To appear in Mem. Amer. Math. Soc., 147 pages, minor typos are
correcte
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