A smooth function changes gradually: similar inputs produce outputs that do not fluctuate wildly. In machine learning, smoothness is one requirement for a useful loss function, alongside specificity and computability. section 4.1.1
Figure 4.2 Examples of a smooth function on the left and two nonsmooth functions on the right. The center example is mostly smooth, but one region is not smooth because the function has no value. On the right, the function is not smooth anywhere due to the hard change in value.
Compare a smooth graph with functions that contain a missing value or a hard change; these disruptions make nearby inputs behave less predictably. section 4.1.1
Investment returns
Investment returns are difficult to predict partly because they are not smooth, illustrating why abrupt variation complicates optimization. section 4.1.1
