  Rectified linear unit (ReLU), as a non-linear activation function, is well
known to improve the expressivity of neural networks such that any continuous
function can be approximated to arbitrary precision by a sufficiently wide
neural network. In this work, we present another interesting and important
feature of ReLU activation function. We show that ReLU leads to: {\it better
separation} for similar data, and {\it better conditioning} of neural tangent
kernel (NTK), which are closely related. Comparing with linear neural networks,
we show that a ReLU activated wide neural network at random initialization has
a larger angle separation for similar data in the feature space of model
gradient, and has a smaller condition number for NTK. Note that, for a linear
neural network, the data separation and NTK condition number always remain the
same as in the case of a linear model. Furthermore, we show that a deeper ReLU
network (i.e., with more ReLU activation operations), has a smaller NTK
condition number than a shallower one. Our results imply that ReLU activation,
as well as the depth of ReLU network, helps improve the gradient descent
convergence rate, which is closely related to the NTK condition number.
