added tw-idf paper and pdf

information_retrieval/README.md

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+## Information Retrieval
+
+Information retrieval is the activity of obtaining information resources relevant to an information need from a collection of information resources. (Says Wikipedia).
+
+The included documents are
+
+*
+  [Graph of Word and TW-IDF](http://www.lix.polytechnique.fr/~rousseau/papers/rousseau-cikm2013.pdf) - Francois Rousseau & Michalis Vazirgiannis
+  
+  The traditional IR system stores term-specific statistics (typically
+  a term's frequency in each document - which we call TF) in an index.
+  Such a model ignores dependencies between terms and considers a
+  document's terms to occur independently of each other (and is aptly
+  called the bag-of-words model). In this paper the authors use a
+  statistic that uses a graph representation of a document to encode
+  dependencies between terms and replace the TF statistic with a new
+  TW statistic based on the graph constructed and achieve
+  significantly better results that popular existing models. This
+  paper won a honorable mention at CIKM 2013.