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For questions about spin manifolds, the groups $\operatorname{Spin}(n)$, as well as generalisations such as $\operatorname{Pin}^{\pm}(n)$ and $\operatorname{Spin}^c(n)$. This tag should also be used for any questions about the geometry of spin manifolds, including questions involving Dirac operators and the Lichnerowicz formula.

16 votes

Pin$^+$ and Pin$^−$ structure for manifolds in any dimensions

The condition for having a $Pin^+$-structure is the vanishing of $w_2$, and for having a $Pin^-$-structure is the vanishing of $w_2 +w_1^2$, for manifolds of any dimension. This is because the Lie gro …
Oscar Randal-Williams's user avatar
12 votes
Accepted

Spin structures and quadratic forms on surfaces

It is not quite true. There is an algebraic gadget one can produce from a Spin structure on a surface with boundary (where we fix the Spin structure along the boundary), and one gets a bijection from …
Oscar Randal-Williams's user avatar
3 votes

Spin structure on mapping torus

You can do this iff the spin structures $\mathfrak{s}$ and $f^*(\mathfrak{s})$ are isomorphic. When $X$ is the 2-torus the set of Spin structures is naturally in bijection with $\mathbb{Z}/2 \oplus \ …
Oscar Randal-Williams's user avatar
2 votes

Injectivity of the $\alpha$-genus

No. See for example the work of Anderson, Brown, and Peterson https://projecteuclid.org/euclid.bams/1183527786
Oscar Randal-Williams's user avatar