  We propose 4-point S-matrices for three-dimensional F-theory. We will use the
twistor formalism to facilitate constructing the amplitude. We write the
amplitude in a way such that the F-symmetry (U-duality symmetry) is manifest.
The amplitude can be schematically written as $A_{4} = w^{4}/stu$, where $w$ is
an analog of the linearized Weyl tensor in F-theory, and $w^{4}$ is a shorthand
for the sum of various contractions that can happen between the Weyl tensors.
The gauge invariance is actually non-trivial since $w$ is in general not gauge
invariant. With the help of the twistor formalism, one can verify that this
formula is indeed gauge invariant. The amplitude also reduces to the ordinary
4-graviton amplitude under the reduction to M-theory (which is just 4D
supergravity).
