Title: Model Architecture Controls Gradient Descent Dynamics: A Combinatorial Path-Based Formula. Abstract: Recently, there has been a growing interest in automatically exploring neural network architecture design space with the goal of finding an architecture that improves performance (characterized as improved accuracy, speed of training, or resource requirements). However, our theoretical understanding of how model architecture affects performance or accuracy is limited. In this paper, we study the impact of model architecture on the speed of training in the context of gradient descent optimization. We model gradient descent as a first-order ODE and use ODE's coefficient matrix H to characterize the convergence rate. We introduce a simple analysis technique that enumerates H in terms of all possible ``paths'' in the network. 
 We show that changes in model architecture parameters reflect as changes in the number of paths and the properties of each path, which jointly control the speed of convergence. We believe our analysis technique is useful in reasoning about more complex model architecture modifications.