"Source paper" of the method Reduction in library Sumy

Does anybody know or can help me to find the source paper of the method Reduction of the library sumy?

This method is here: Reduction where it says that it is based on here, but in none of these places I am able to find the source.

At this description, it only says that: Graph-based summarization, where a sentence salience is computed as the sum of the weights of its edges to other sentences. The weight of an edge between two sentences is computed in the same manner as TextRank.

The entire library is said to be based on this thesis. However, it is in germany. I tried to find, but I was not able to do that because of language barrier.