AdaGrad adapts the learning rate separately for each parameter dimension. This addresses asymmetric loss surfaces, where one shared learning rate may make steep dimensions oscillate while leaving shallow dimensions slow to progress. section 9.2.7 section 9.0
It maintains a state vector containing the cumulative sum of squared partial derivatives for every dimension. Historically large gradients produce larger accumulated values and therefore smaller effective learning rates; historically small gradients retain larger effective learning rates. A small ε prevents division by zero, after which weights and biases receive the adjusted update. section 9.2.7
A speaking-time analogy
AdaGrad gives less attention to dimensions that have produced large slopes repeatedly and more attention to dimensions that have changed infrequently—like paying less attention to someone who speaks constantly and more to someone who rarely speaks. section 9.2.7
Equation 9.14
Use the equation figure to inspect AdaGrad’s accumulated-gradient state and per-dimension update. section 9.2.7

