Spinc structure

In spin geometry, a spin^c structure (or complex spin structure) is a special classifying map that can exist for orientable manifolds. Such manifolds are called spin^c manifolds. C stands for the complex numbers, which are denoted ${\displaystyle \mathbb {C} }$ and appear in the definition of the underlying spin^c group. In four dimensions, a spin^c structure defines two complex plane bundles, which can be used to describe negative and positive chirality of spinors, for example in the Dirac equation of relativistic quantum field theory. Another central application is Seiberg–Witten theory, which uses them to study 4-manifolds.

Let ${\displaystyle M}$ be a ${\displaystyle n}$-dimensional orientable manifold. Its tangent bundle ${\displaystyle TM}$ is described by a classifying map ${\displaystyle M\rightarrow \operatorname {BSO} (n)}$ into the classifying space ${\displaystyle \operatorname {BSO} (n)}$ of the special orthogonal group ${\displaystyle \operatorname {SO} (n)}$. It can factor over the map ${\displaystyle \operatorname {BSpin} ^{\mathrm {c} }(n)\rightarrow \operatorname {BSO} (n)}$ induced by the canonical projection ${\displaystyle \operatorname {Spin} ^{\mathrm {c} }(n)\twoheadrightarrow \operatorname {SO} (n)}$ on classifying spaces. In this case, the classifying map lifts to a continuous map ${\displaystyle M\rightarrow \operatorname {BSpin} ^{\mathrm {c} }(n)}$ into the classifying space ${\displaystyle \operatorname {BSpin} ^{\mathrm {c} }(n)}$ of the spin^c group ${\displaystyle \operatorname {Spin} ^{\mathrm {c} }(n)}$, which is called spin^c structure.

Let ${\displaystyle \operatorname {BSpin} ^{\mathrm {c} }(M)}$ denote the set of spin^c structures on ${\displaystyle M}$ up to homotopy. The first unitary group ${\displaystyle \operatorname {U} (1)}$ is the second factor of the spin^c group and using its classifying space ${\displaystyle \operatorname {BU} (1)\cong \operatorname {BSO} (2)}$, which is the infinite complex projective space ${\displaystyle \mathbb {C} P^{\infty }}$ and a model of the Eilenberg–MacLane space ${\displaystyle K(\mathbb {Z} ,2)}$, there is a bijection:

Due to the canonical projection ${\displaystyle \operatorname {BSpin} ^{\mathrm {c} }(n)\rightarrow \operatorname {U} (1)/\mathbb {Z} _{2}\cong \operatorname {U} (1)}$, every spin^c structure induces a principal ${\displaystyle \operatorname {U} (1)}$-bundle or equvalently a complex line bundle.

The following properties hold more generally for the lift on the Lie group ${\displaystyle \operatorname {Spin} ^{k}(n):=\left(\operatorname {Spin} (n)\times \operatorname {Spin} (k)\right)/\mathbb {Z} _{2}}$, with the particular case ${\displaystyle k=2}$ giving: