Math
Logic
Topology
Galois's theory
Information theory
The information is measured using the following formula
$$ I(p) = \log_2\left(1\over p\right) $$
It's possible to define the Entropy for a system using an alphabet of \(q\) simbols with probabilities \(p_i\) as the average amount of information
$$ H(P) = \sum_{i=1}^q p_i\log\left(1\over p_i\right) $$
It's possible to demonstrate that the maximum value for \(H\) is obtained with symbols having all the same probabilities; moreover it's possible to show that
$$ H(P) \leq L = \hbox{average code length} $$