Mathematics
The Rank-Nullity Theorem and the Dimensions of Linear Maps
Quick fact
For any linear map from a finite-dimensional vector space to another, the dimension of the domain is exactly the sum of the dimensions of its kernel and its image—this is the rank-nullity theorem.
Why this is interesting
What if a linear map 'squishes' a whole line of points into a single point? How much information is lost, and can we still know the dimension of the original space?