In many cases an activation function is notated as g (e.g. Andrew Ng's Course courses), especially if it doesn't refer to any specific activation function such as sigmoid.

However, where does this convention come from? And for what reason did g start to be used?

  • 1
    $\begingroup$ It’s just a notation. You can call it whatever you want. a BC d $\endgroup$ Nov 3, 2017 at 12:25
  • $\begingroup$ Yes but what is the reason? That is the point of my question. $\endgroup$
    – Blaszard
    Nov 3, 2017 at 13:12
  • $\begingroup$ Read some linear algebra. $\endgroup$ Nov 3, 2017 at 13:30
  • 2
    $\begingroup$ That is not an answer. $\endgroup$
    – Blaszard
    Nov 6, 2017 at 10:03
  • $\begingroup$ ESL uses $\sigma(.)$. This is a dumb question. $\endgroup$ Nov 6, 2017 at 21:37

1 Answer 1


The addition of the activation layer creates a composition of two functions.

"A general function, to be defined for a particular context, is usually denoted by a single letter, most often the lower-case letters f, g, h."

So it comes down to the reason that he uses the hypothesis representation h(x)=wX+b which is a function, and that is wrapped by an activation function denoted as g. The choice of g seems to be purely alphabetical.

  • $\begingroup$ He didn't write the hypothesis function as h; he used z. So it was not intuitive for me why he used g... $\endgroup$
    – Blaszard
    Nov 9, 2017 at 19:28

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