For some reason that I do not fully understand, the equality of cross partial derivatives gets all munged up, in naming at least, when taught in the typical math econ class, including the one when I was an undergraduate student. In reading this great expository blog on mathematics, I finally found a reference to a named theorem about it. And the name is not Young’s Theorem, as I have been taught. I learn something every day. Wikipedia has something to say about this, too.