1. D. Fox, 𝑘-to-1 continuous transformations, University of California at Riverside Dissertation, August 1973.
2. On 𝑘-to-1 transformations;Gottschalk, W. H.;Bull. Amer. Math. Soc.,1947
3. The non-existence of a certain type of continuous transformation;Harrold, O. G., Jr.;Duke Math. J.,1939
4. There is no exactly 𝑘-to-1 function from any continuum onto [0,1], or any dendrite, with only finitely many discontinuities;Heath, Jo W.;Trans. Amer. Math. Soc.,1988
5. Every exactly 2-to-1 function on the reals has an infinite set of discontinuities;Heath, Jo;Proc. Amer. Math. Soc.,1986