Wao thank you very very much ! You understand exactly what I need ! I am going to read this article carefully !

Yes, Ai has probability pi, and you write well about the n symbols and about the sequence Xk. But the source is variable. That means that it transmits (A1, A2, A3, A4, A5, ... Aj) with j≤n. And when its entropy H(X)>C, it doesn't transfer as much information as it needs to reduce its entropy. That is to mean, it self-manages entropy to maintain H(X)<C. For this, each Ai is assigned a "coefficient of importance" λi. And the source no longer transfers information Ai that has a small λi. If you can help me to describe this source with the standard terminology thank you very much