Do diffeomorphisms have to be one-to-one functions?

Therefore, for a diffeomorphism, the original function must be one-to-one to ensure its inverse is a function. However, the inverse can also be a relation, but it may not have all the properties of a function. In summary, for a diffeomorphism, the original function must be one-to-one to guarantee its inverse is a function, but it can also be a relation.