  We present an extended, thematically reinforced version of Gabrilovich and
Markovitch's Explicit Semantic Analysis (ESA), where we obtain thematic
information through the category structure of Wikipedia. For this we first
define a notion of categorical tfidf which measures the relevance of terms in
categories. Using this measure as a weight we calculate a maximal spanning tree
of the Wikipedia corpus considered as a directed graph of pages and categories.
This tree provides us with a unique path of "most related categories" between
each page and the top of the hierarchy. We reinforce tfidf of words in a page
by aggregating it with categorical tfidfs of the nodes of these paths, and
define a thematically reinforced ESA semantic relatedness measure which is more
robust than standard ESA and less sensitive to noise caused by out-of-context
words. We apply our method to the French Wikipedia corpus, evaluate it through
a text classification on a 37.5 MB corpus of 20 French newsgroups and obtain a
precision increase of 9-10% compared with standard ESA.
