This paper presents a general theory of one-dimensional differentiability for functionals having convex domains which are not necessarily open. The local approximation is carried out by an affine functions, rather than a linear one as in the standard Gateaux derivative. These derivatives have many applications but their theoretical properties are underexplored in literature. We also analyze variations such the affine counterpart of the classical Hadamard and Frechet derivatives. Applications to Statistics and Economics are also discussed.
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