Given a Poisson algebra, a deformation quantization is an associative unital product ★ on the algebra of formal power series in, subject to the following two axioms, If one were given a Poisson manifold, one could ask, in addition, that where the are linear bidifferential operators of degree at most. Two deformations are said to be equivalent iff they are related by a gauge transformation of the type, where are differential operators of order at most. The corresponding induced ★-product, ★′, is then For the archetypal example, one may well consider Groenewold's original "Moyal-Weyl" ★-product.
Kontsevich graphs
A Kontsevich graph is a simple directed graph without loops on 2 external vertices, labeled f and g; and internal vertices, labeled. From each internal vertex originate two edges. All graphs with internal vertices are accumulated in the set. An example on two internal vertices is the following graph,
Associated bidifferential operator
Associated to each graph, there is a bidifferential operator defined as follows. For each edge there is a partial derivative on the symbol of the target vertex. It is contracted with the corresponding index from the source symbol. The term for the graph is the product of all its symbols together with their partial derivatives. Here f and g stand for smooth functions on the manifold, and is the Poisson bivector of the Poisson manifold. The term for the example graph is
Associated weight
For adding up these bidifferential operators there are the weights of the graph. First of all, to each graph there is a multiplicity which counts how many equivalent configurations there are for one graph. The rule is that the sum of the multiplicities for all graphs with internal vertices is. The sample graph above has the multiplicity. For this, it is helpful to enumerate the internal vertices from 1 to. In order to compute the weight we have to integrate products of the angle in the upper half-plane, H, as follows. The upper half-plane is, endowed with a metric and, for two points with, we measure the angle between the geodesic from to and from to counterclockwise. This is The integration domain is Cn the space The formula amounts where t1 and t2 are the first and second target vertex of the internal vertex. The vertices f and g are at the fixed positions 0 and 1 in.
The formula
Given the above three definitions, the Kontsevich formula for a star product is now
