Mathematics
The Rank-Nullity Theorem
Quick fact
For any linear map from one finite-dimensional vector space to another, the dimension of its kernel plus the dimension of its image always equals the dimension of its domain—no matter what the map is.
Why this is interesting
Have you ever wondered how many independent directions a transformation can ‘reach’ versus how many it ‘collapses’? The rank-nullity theorem reveals a beautiful balance between these two numbers.