diff --git a/symptom_ex_process/algorithm.tex b/symptom_ex_process/algorithm.tex
index a4ec030..b8465c6 100644
--- a/symptom_ex_process/algorithm.tex
+++ b/symptom_ex_process/algorithm.tex
@@ -19,7 +19,7 @@ However, it can still be treated
 as a component with a known set of failure modes.
 \paragraph{Enumerating abstraction levels}
 We can assign an attribute  of abstraction level to
-components $\alpha$, where $\alpha$ is a natural number, ($\alpha \in \mathbb{N}$).
+components $\alpha$, where $\alpha$ is a natural number, ($\alpha \in \mathbb{N}_0$).
 For a base component let the abstraction level be zero.
 If we apply the symptom abstraction process $\bowtie$
 the resulting derived~component will have an $\alpha$ value