https://latex.codecogs.com/png.latex?%0A%5Clabel%7Beq%3Asgd.saddle%7D%0A%5CDelta%20E%20%3D%20%5Cfrac%7B%5Cpartial%20f%7D%7B%5Cpartial%20x%7D%20%5CDelta%20x%20%2B%20%5Cfrac%7B%5Cpartial%20f%7D%7B%5Cpartial%20y%7D%20%5CDelta%20y%20%3D%202x%20%5CDelta%20x%20-%202y%20%5CDelta%20y%5C%3B%5Censpace%5Cmathrm%7Bwhere%7D%5Censpace%20E%28x%2Cy%29%20%3D%20x%5E2%20-%20y%5E2.%0A%5Cqquad%2876%29