Norm-Cone Conjugation and Fenchel-Type Duality Beyond Convexity

We introduce a norm-cone conjugation scheme for extended-real-valued functions on normed spaces. The construction replaces affine minorants by translated norm-cones of the form $x\mapsto r-\alpha\|x-x_0\|$, with $\alpha\ge0$, and establishes a nonlinear conjugation framework underlying a Fenchel-type duality theory beyond convexity. The resulting conjugate is indexed by slopes and centres, and the associated biconjugate is the supremum of all norm-cone minorants lying below the function. We prove Fenchel--Young type inequalities, introduce admissible slopes and admissible heights, and characterize exact biconjugation in terms of norm-cone supportability. We also define a norm-cone subdifferential and relate it to exact support and biconjugation. Finally, we develop an abstract perturbation duality theory based on partial norm-cone conjugation in the perturbation variable. Weak duality holds without convexity assumptions, while strong duality follows from metric lower-bound conditions, including uniform lower Lipschitz estimates and lower calmness of the value function.