# Degree of a mapping

degree of a continuous mapping between connected compact manifolds of identical dimension

An integer such that , where are the fundamental classes (cf. Fundamental class) of the manifolds and over the ring or , and is the induced mapping. In the case of non-orientable manifolds, the degree of the mapping is uniquely defined modulo 2. If is a differentiable mapping between closed differentiable manifolds, then modulo 2 coincides with the number of inverse images of a regular value of . In the case of oriented manifolds where is the sign of the Jacobian of at a point (the Browder degree).

For a continuous mapping and an isolated point in the inverse image of zero, the concept of the local degree at the point is defined: , where is the restriction of onto a small sphere and is the projection from zero onto the unit sphere. In the case of a differentiable , the formula holds, where is the ring of germs (cf. Germ) of smooth functions at zero, factorized by the ideal generated by the components of , and is the maximal ideal of the quotient ring relative to the property . Let be the class of the Jacobian of the mapping . Then for a linear form such that the formula holds, where is a symmetric bilinear form on .