2.1 Norms and distances
Previously, we saw that vectors are essentially arrows, starting from the null vector. In addition to their direction, vectors also have magnitude. For example, as we have learned in high school mathematics, the magnitude in the Euclidean plane is defined by
while we can calculate the distance between x and y as
(The function ∥⋅∥ simply denotes the magnitude of a vector.)
The magnitude formula
can be simply generalized to higher dimensions by
However, just from looking at this formula, it is not clear why it is defined this way. What does the square root of a sum of squares have to do with distance and magnitude? Behind the scenes, it is just the Pythagorean theorem.
Recall that the Pythagorean theorem states that in right triangles, the squared length of the hypotenuse equals the sum of the squared lengths of the other sides, as illustrated by Figure 2.3.