Downcase n in formula

natural-language.tex

@@ -53,7 +53,7 @@
 
 \begin{definition}[Inverse document frequency]\label{def:idf}
   Calculated for a document collection $D = \{ d_1, \dots, d_n \}$:
-  \[ \mathrm{idf}(t, D) = \log_{10} \left(\frac{N}{|\{ d \in D | t \in
+  \[ \mathrm{idf}(t, D) = \log_{10} \left(\frac{n}{|\{ d \in D | t \in
         d \}|}\right) \]
 \end{definition}