Add bbl to avoid need to run biber

Signed-off-by: Marcello Seri <[email protected]>

.gitignore

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 aom.pdf
 
 ## Bibliography auxiliary files (bibtex/biblatex/biber):
-*.bbl
 *.bcf
 *.blg
 *-blx.aux

7-integration.tex

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+We finally have all the main ingredients to generalize our line integral detour and discuss integration of $n$-forms over $n$-dimensional manifolds.
+
 \section{Orientation}
+
+\newthought{We know from calculus one}, or our line integral examples, that the direction in which we traverse the interval, or a curve, can actually make a difference.
+Indeed, the sign of the integral of a differential $n$-form is only fixed after choosing an orientation of the manifold.
+
+If for a curve an orientation is simply a choice of a direction along it, so we can make sense of it in terms of clockwise or counter-clockwise, generalising the concept will require an extra abstraction step.
+Not just that, you have seen already that in $\R^n$ there is a standard orientation, but in other vector spaces we may need to make arbitrary choices.
+For manifolds, the situation is much more complicated: for example, on a M\"obius strip it is impossible to make any such choice, as it turns out, it is non-orientable.
+
 \section{Stokes' Theorem}