http://science.jrank.org/pages/2881/Function-History-functions.html
https://www.cut-the-knot.org/do_you_know/FunctionMain.shtml
http://www.storyofmathematics.com/mathematicians.html
https://faculty.math.illinois.edu/~castelln/M117/lecture8_math117.pdf
http://www-history.mcs.st-and.ac.uk/HistTopics/Functions.html
https://pdfs.semanticscholar.org/80db/007da68b6884b686890c1ba58f21800ed647.pdf
