Busemann-coupling subdifferentials and Fenchel biconjugation on Hadamard manifolds
We introduce an intrinsic subdifferential on Hadamard manifolds induced by a base-point-dependent Busemann coupling, motivated by the scarcity of nonconstant affine functions on non-Euclidean Hadamard manifolds. For proper functions, it characterizes exactly the primal-dual equality pairs in the Fenchel-Young inequality of Bento, Cruz Neto, and Melo (\emph{Appl. Math. Optim.} 88:83, 2023). A dual-point--tangent-vector bijection yields Fermat's rule and isometry covariance. We characterize everywhere nonemptiness for finite-valued functions by pointwise-attained \(H_p\)-envelope representations, encompassing the radial models with explicit subdifferentials and the nonsmooth distance from the base point. In hyperbolic space, base-point rigidity shows that, for functions differentiable at the base point, nonemptiness is equivalent to global minimality and vanishing of the corresponding Fenchel-Young gap. Two geodesically convex functions on the Poincaré disk prove strict noninclusions between our construction and fixed-direction Busemann subdifferentials, although both recover the classical Euclidean subdifferential. This separation is explained by coupling orientation. In constant negative curvature, a strictly increasing scalar profile describes the asymmetry and defines an intrinsic measure of nonlinearity. We compute this measure at arbitrary radius and derive a lower bound and two-sided estimates for the biconjugation gap; at unit radius, the exact curvature-dependent bound is attained by deviations of both signs.