  The logical connectivity of text is represented by the connectivity of words
that form archipelagoes. Here, each archipelago is a sequence of islands of the
occurrences of a certain word. An island here means the local sequence of
sentences where the word is emphasized, and an archipelago of a length
comparable to the target text is extracted using the co-variation of entropy A
(the window-based entropy) on the distribution of the word's occurrences with
the width of each time window. Then, the logical connectivity of text is
evaluated on entropy B (the graph-based entropy) computed on the distribution
of sentences to connected word-clusters obtained on the co-occurrence of words.
The results show the parts of the target text with words forming archipelagoes
extracted on entropy A, without learned or prepared knowledge, form an
explanatory part of the text that is of smaller entropy B than the parts
extracted by the baseline methods.
