Added facts about vector measures.

src/measure/vector/ac.tex

@@ -0,0 +1,10 @@
+\section{Absolutely Continuous}
+\label{section:absolutely-continuous-measure}
+
+\begin{definition}[Absolutely Continuous]
+\label{definition:absolutely-continuous}
+    Let $(X, \cm)$ be a measurable space and $\mu, \nu$ be signed/vector measures on $X$, then $\nu$ is \textbf{absolutely continuous} with respect to $\mu$, denoted $\nu \ll \mu$, if every $\mu$-null set is $\nu$-null.
+\end{definition}
+
+
+