Figure 1. Projecting a paper citation relationship into a city-to-city citation network.

(A) Paper A written by authors from Ann Arbor, Los Alamos and New York cites one paper B written by authors from Rome and Madrid and another paper C from Oxford and Princeton. (B) In a city-to-city citation network, directed links from Ann Arbor to Madrid, Rome, Oxford and Princeton are generated, and similarly Los Alamos and New York are connected to the above four cited cities.