diff --git a/latex/thesis/chapters/theoretical_background.tex b/latex/thesis/chapters/theoretical_background.tex
index 8db32f1..200aca7 100644
--- a/latex/thesis/chapters/theoretical_background.tex
+++ b/latex/thesis/chapters/theoretical_background.tex
@@ -252,6 +252,8 @@ For the code depicted in figure \ref{fig:theo:tanner_graph}, for example,
 $N_c\left( 1 \right) = \left\{ 1, 3, 5, 7 \right\}$ and
 $N_v\left( 3 \right) = \left\{ 1, 2 \right\}$.
 
+\todo{Define $d_i$ and $d_j$}
+
 Message passing algorithms are based on the notion of passing messages between
 \acp{CN} and \acp{VN}.
 \Ac{BP} is one such algorithm that is commonly used to decode \ac{LDPC} codes.