Anomaly of 4d Weyl fermions with discrete symmetries

We derive explicit anomaly-index formulas for four-dimensional Weyl fermions charged under the finite symmetries $\mathrm{Spin}\times\mathbb Z_n$ and $\mathrm{Spin}\times_{\mathbb Z_2^{\mathrm F}}\mathbb Z_{2m}^{\mathrm F}$. The strategy is to start from the standard perturbative anomaly indices for $\mathrm{Spin}\times\mathrm U(1)$ and $\mathrm{Spin}\times_{\mathbb Z_2^{\mathrm F}}\mathrm U(1)=\mathrm{Spin}^c$, and then restrict the continuous $\mathrm U(1)$ symmetry to a finite cyclic subgroup. On the level of invertible field theories this gives natural homomorphisms $$\mathrm{TP}_5(\mathrm{Spin}\times\mathrm U(1)) \longrightarrow \mathrm{TP}_5(\mathrm{Spin}\times\mathbb Z_n),\quad \mathrm{TP}_5(\mathrm{Spin}^c) \longrightarrow \mathrm{TP}_5(\mathrm{Spin}\times_{\mathbb Z_2^{\mathrm F}}\mathbb Z_{2m}^{\mathrm F}).$$ We compute these maps explicitly by evaluating reduced $\eta$-invariants on geometric representatives of the finite anomaly groups. For $\mathrm{Spin}\times\mathbb Z_n$, the relevant backgrounds are the five-dimensional lens-space bundle $X(n;1,1)$ and the product $L(n;1)\times\mathrm{K3}$. For $\mathrm{Spin}\times_{\mathbb Z_2^{\mathrm F}}\mathbb Z_{2m}^{\mathrm F}$, the relevant backgrounds are $L(m;1,1,1)$ and, depending on the parity of $m$, either $L(m;1)\times\mathrm{Enriques}$ or $L(m;1)\times\mathrm{K3}$. The output is a pair of integer-valued anomaly indices for each finite symmetry. These indices are normalized in the cyclic factors of the finite anomaly group, so they can be used directly in anomaly-cancellation checks for fermions with discrete gauge or global symmetries.