Consider the softmax function $f \colon \mathbb{R}^k \rightarrow \mathbb{R}^k $ defined by $f((x_1, \ldots, x_n)) = (y_1, \ldots, y_n)$ where $y_i = \frac{e^{x_i}}{\sum_j e^{x_j}}$.

Can you show that for every $x, \Delta y,\delta$, there exists $\epsilon$ and $\Delta x < \delta$ such that $f(x + \Delta x) = f(x) + \epsilon \cdot \Delta y$? Also, is there a name for this property for general function? Assume there exists $x'$ such that $y + \Delta y = f(x')$.


I'm not so sure that you can make that statement. By the construction of the softmax, the sum of its entries is equal to $1$, in other words:

$$\sum_{i=1}^k [f(x + \Delta x)]_i = 1$$


$$\sum_{i=1}^k [f(x)]_i = 1$$

However, from what you've written,

$$ f(x+\Delta x) = f(x) + \epsilon \Delta y $$

It would imply that

$$\sum_{i=1}^k [f(x) + \epsilon \Delta y]_i > 1$$

which leads us to a contradiction if we sum over the entries of $f(x + \Delta x)$:

$$ \sum_{i=1}^k [f(x+\Delta x)]_i = \sum_{i=1}^k [f(x) + \epsilon \Delta y]_i > 1 $$

  • $\begingroup$ updated the question. I meant for any valid vector $\Delta y$. $\endgroup$ – listener Feb 1 at 5:39

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