- Serre's multiplicity conjectures
In
mathematics , Serre's multiplicity conjectures are certain purely algebraic problems, incommutative algebra , motivated by the needs ofalgebraic geometry . SinceAndré Weil 's initial rigorous definition ofintersection number s, around 1949, there had been a question of how to provide a more flexible and computable theory.Let "R" be a (Noetherian, commutative)
regular local ring and "P" and "Q" beprime ideal s of "R". In 1961,Jean-Pierre Serre realized that classical algebraic-geometric ideas of multiplicity could be generalized using the concepts ofhomological algebra . Serre defined theintersection multiplicity of "R/P" and "R/Q" by means of theTor functors ofhomological algebra , as:chi (R/P,R/Q):=sum _{i=0}^{infty}(-1)^iell_R (Tor ^R_i(R/P,R/Q)).
This requires the concept of the
length of a module , denoted here by "lR", and the assumption that:ell _R((R/P)otimes(R/Q)) < infty.
If this idea were to work, however, certain classical relationships would presumably have to continue to hold. Serre singled out four important properties. These then became conjectures, challenging in the general case.
Dimension inequality
: dim(R/P) + dim(R/Q) le dim(R)
Serre verified this for all regular local rings. He established the following three properties when "R" is
unramified , and conjectured that they hold in general.Nonnegativity
: chi (R/P,R/Q) ge 0
Ofer Gabber verified this, quite recently.Vanishing
If
: dim (R/P) + dim (R/Q) < dim (R)
then
: chi (R/P,R/Q) = 0.
This was proven around 1986 by
Paul C. Roberts , and independently by Gillet and Soulé.Positivity
If
: dim (R/P) + dim (R/Q) = dim (R)
then
: chi (R/P,R/Q) > 0.
This remains open.
References
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