-Stack Overflow, "What is a Y-combinator?": https://stackoverflow.com/questions/93526/what-is-a-y-combin...
-Wikipedia, "Fixed-point combinator," at the Y combinator sub-section: https://en.wikipedia.org/wiki/Fixed-point_combinator#Y_combi...
-For a more academic source, an article from a Johns Hopkins computer science professor is at: https://www.cs.jhu.edu/~jason/465/readings/lambdacalc.html