In (4.1.5), W_1 * X should be X * W_1, W_2 * H_1 should be H_1 *W_2, and W_3 * H_2 should be H_2 * W_3.

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Agreed, if they stick to the convention of one input sample per row I think this is the only way.

In fact in the previous chapter the **Vectorization for Minibatches** of the **softmax regression** section you can see that the linear computation for the softmax is XW + b even though there is another mistake in the explanation where again it is shown WX.

Has anyone figured out exercise 4 or 5?

Can someone please explain the \mathcal{O}(n^{-\frac{1}{2}}) ?

### 4.4.2.1. Validation Dataset

The uncertainty in our estimates can be shown to be of the order of \mathcal{O}(n^{-\frac{1}{2}}).