  In this paper we study the minimal norm tensors for general third covariant
tensors and fourth covariant tensors, using this we can give a new explanation
of Weyl tensor and Cotten tensor: Weyl tensor is the minimal norm tensor of
Riemannian curvature tensor and Cotten tensor is the minimal norm tensor of
divergence of Riemannian curvature tensor, and we also get some useful
inequalities by computation the norm of minimal norm tensors.
