  Technological innovation has extensively been studied to make firms
sustainable and more competitive. Within this context, the most important
recent issue has been the dynamics of collaborative innovation among firms. We
therefore investigated a patent network, especially focusing on its spatial
characteristics. The results can be summarized as follows. (1) The degree
distribution in a patent network follows a power law. A firm can then be
connected to many firms via hubs connected to the firm. (2) The neighbors'
average degree has a null correlation, but the clustering coefficient has a
negative correlation. The latter means that there is a hierarchical structure
and bridging different modules may shorten the paths between the nodes in them.
(3) The distance of links not only indicates the regional accumulations of
firms, but the importance of time it takes to travel, which plays a key role in
creating links. (4) The ratio of internal links in cities indicates that we
have to consider the existing links firms have to facilitate the creation of
new links.
