We discuss the effective theory of F-theory compactifications by using the M-theory dual. Two main results are highlighted: (1) the chiral index of a 4D F-theory compactification can be determined by four-form flux when comparing 3D Chern-Simons theories at one loop, (2) the inclusion of warping effects due to fluxes in M-theory is crucial to determine corrections to the F-theory effective theory even if in the latter warping effects can be neglected.
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