Formal notions of function, which appear in middle school, are discussed in light of how teachers might complement the input-output notion with a covariation perspective.
Edited by Amy Ellis, aellis1@education.wisc.edu, and Despina Stylianou, dstylianou@ccny.cuny.edu. Readers are encouraged to submit manuscripts through http://mtms.msubmit.net.
Eric Weber, eric.weber@oregonstate.edu, is an assistant professor of mathematics education at Oregon State University in Corvallis, Oregon. He is interested in student learning about functions and rates of change.