Title: Implicit Jacobian regularization weighted with impurity of probability output. Abstract: Gradient descent (GD) plays a crucial role in the success of deep learning, but it is still not fully understood how GD finds minima that generalize well. In many studies, GD has been understood as a gradient flow in the limit of vanishing learning rate. However, this approach has a fundamental limitation in explaining the oscillatory behavior with iterative catapult in a practical finite learning rate regime. To address this limitation, we rather start with strong empirical evidence of the plateau of the sharpness (the top eigenvalue of the Hessian) of the loss function landscape. With this observation, we investigate the Hessian through simple and much lower-dimensional matrices. In particular, to analyze the sharpness, we instead explore the eigenvalue problem for the low-dimensional matrix which is a rank-one modification of a diagonal matrix. The eigendecomposition provides a simple relation between the eigenvalues of the low-dimensional matrix and the impurity of the probability output. We exploit this connection to derive sharpness-impurity-Jacobian relation and to explain how the sharpness influences the learning dynamics and the generalization performance. In particular, we show that GD has implicit regularization effects on the Jacobian norm weighted with the impurity of the probability output.