chrishkchris commented on issue #488: SINGA -475 add Sign operator to singa URL: https://github.com/apache/incubator-singa/pull/488#issuecomment-519856801 In my opinion: For the function y=sign(x) , y = 1 when x > 0, y = -1 when x < 0. Therefore, the derivative is 0 except x = 0. The function y=sign(x) is discontinuous at x=0 where the derivative is undefined in the mathematical logic. However, an defined derivative leads to a nan number, so we used a derivative of zero also at x=0. As a result, dx is always 0 while the output shape (size of array) is the same as input shape.
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