  Let $G(n)=\sigma (n)/(n \log \log n )$. Robin made hypothesis that
$G(n)<e^\gamma$ for all integer $n>5040$. This article divides all colossally
abundant numbers in to three disjoint subsets CA1, CA2 and CA3, and shows that
Robin hypothesis is true if and only if all CA2 numbers $n>5040$ satisfy Robin
inequality.
