A Function-Centric Perspective on Flat and Sharp Minima

Flat minima are strongly associated with improved generalisation in deep neural networks. However, this connection has proven nuanced in recent studies, with both theoretical counterexamples and empirical exceptions emerging in the literature. In this paper, we revisit the role of sharpness in model performance and argue that sharpness is better understood as a function-dependent property rather than an indicator of poor generalisation. We conduct extensive empirical studies ranging from single-objective optimisation, synthetic non-linear binary classification tasks, to modern image classification tasks. In single-objective optimisation, we show that flatness and sharpness are relative to the function being learned: equally optimal solutions can exhibit markedly different local geometry. In synthetic non-linear binary classification tasks, we show that increasing decision-boundary tightness can increase sharpness even when models generalise perfectly, indicating that sharpness is not reducible to memorisation alone. Finally, in large-scale experiments, we find that sharper minima often emerge when models are regularised (e.g., via weight decay, data augmentation, or SAM), and coincide with better generalisation, calibration, robustness, and functional consistency. Our findings suggest that function complexity, rather than flatness, shapes the geometry of solutions, and that sharper minima can reflect more appropriate inductive biases, calling for a function-centric reappraisal of minima geometry.