Cezar Joita, Robin Chapman. The Functional Equation f g = f: 10818. The American Mathematical Monthly, 109(5):479-480, 2002. [doi]
@article{JoitaC02, title = {The Functional Equation f g = f: 10818}, author = {Cezar Joita and Robin Chapman}, year = {2002}, url = {http://www.jstor.org/stable/2695662}, researchr = {https://researchr.org/publication/JoitaC02}, cites = {0}, citedby = {0}, journal = {The American Mathematical Monthly}, volume = {109}, number = {5}, pages = {479-480}, }