Williams, Peter (Aug 21, 1996). "Proving Functions One-to-One". Department of Mathematics at CSU San Bernardino Reference Notes Page. Archived from the original on 4 June 2017.
Williams, Peter (Aug 21, 1996). "Proving Functions One-to-One". Department of Mathematics at CSU San Bernardino Reference Notes Page. Archived from the original on 4 June 2017.