These haven't been run in awhile; I updated them so they
have the same look and feel as the book, and fixed a few
imports and pep 8 compliance as well.
I moved the conversion of the multivariate equations to univariate
equations to the supporting textbook. It's not terribly necessary,
especially since I converted the univariate equations to look like
the multivariate ones.
Some equations used \\ without a gathered or aligned block.
They render fine in the notebook, not not in the PDF.
Also, switched back my ill chosen use of \overline for \bar.
