2 research outputs found
The graded Betti numbers of truncation of ideals in polynomial rings
Let , a graded algebra satisfies
if is generated in degree , and the graded minimal resolution
is linear the first steps, and the -index of is the largest such
that satisfies . Eisenbud and Goto have shown that for any graded
ring , then , where and
, has a -linear resolution (satisfies for all
) if . For a squarefree monomial ideal , we are here interested in
the ideal which is the squarefree part of . The ideal is,
via Stanley-Reisner correspondence, associated to a simplicial complex
. In this case, all Betti numbers of for
, which of course is a much finer invariant
than the index, can be determined from the Betti diagram of and the
-vector of . We compare our results with the corresponding
statements for . (Here is an arbitrary graded ideal.) In this
case we show that the Betti numbers of can be determined from the
Betti numbers of and the Hilbert series of
On the Eliahou and Villarreal conjecture about the projective dimension of co-chordal graphs
Let be the edge ideal of a graph with and
be a polynomial ring in variables over a
field . In this paper we are interested in a conjecture of Eliahou
and Villarreal which states that when is connected and co-chordal. We
show that this conjecture is not true in general. In fact we show that the
difference between and is not necessarily bounded. For any graph
we prove that . For a non-increasing sequence of positive integers
, we define the -tree graphs. We show
that the independence complex of the complements of these type of trees is
vertex decomposable and quasi-forest. Finally we show that the conjecture is
valid when the complement of is a -tree or has a
full-vertex. To our knowledge the results in this paper generalise all the
existing classes of graphs for which the conjecture is true.Comment: 15 pages, comments are welcom