Invertible function without onto I have come across a situation in differential calculus where my mentor said that a function is invertible if it is one one because we consider range of function as domain of function's inverse. What does it mean?

When we restrict the codomain to the range of the function, then of course, the term onto can be neglected, and only by checking the one one condition, we can say whether the given function is invertible or not. Onto is only needed to be considered, when the image set i.e. range is not equal to the codomain.
