IEEE TRANSACTIONS ON NEURAL NETWORKS, VOL. XX, NO. Y, MONTH, YEAR 3
Traditional machine learning applications cope with graph structured data by using a preprocessing phase which
maps the graph structured information to a simpler representation, e.g. vectors of reals. In other words, the
preprocessing step first “squashes” the graph structured data into a vector of reals and then deal with the preprocessed
data using a list based data processing technique. However, important information, e.g., the topological dependency
of information on node n may be lost during the preprocessing stage and the final result may depend, in an
unpredictable manner, on the details of the preprocessing algorithm. More recently, there are various approaches
[14], [15] attempting to preserve the graph structured nature of the data for as long as required before processing
the data. In these recent approaches the idea is to encode the underlying graph structured data using the topological
relationships among the nodes of the graph to incorporate the graph structured information in the data processing
step. Recursive neural networks [14], [16], [17] and Markov chains [15], [18], [19] belong to this set of techniques
and are commonly applied both to graph and node focused problems. Our method extends these two approaches
in that it can deal directly with graph structured information.
Existing recursive neural networks are neural network models whose input domain consists of directed acyclic
graphs [14], [16], [17]. The method estimates the parameters ϕ
w
of a function which maps a graph to a vector
of reals. The approach can also be used for node focused applications, but in this case, the graph must undergo
a preprocessing phase [20]. Similarly, using a preprocessing phase, it is possible to handle certain types of cyclic
graphs [21]. Recursive neural networks have been applied to several problems including logical term classification [22],
chemical compound classification [23], logo recognition [2], [24], web Page scoring [25], and face localization [26].
Recursive neural networks are also related to support vector machines [27], [28], [29], which adopt special
kernels to operate on graph structured data. For example, the diffusion kernel [30] is based on a heat equation; the
kernels proposed in [31], [32] exploit the vectors produced by a graph random walker and those designed in [33],
[34], [35] use a method of counting the number of common substructures of two trees. In fact, recursive neural
networks, as support vector machine methods, automatically encode the input graph into an internal representation.
However, in recursive neural networks, the internal encoding is learned, while in support vector machine it is
designed by the user.
On the other hand, Markov chain models can emulate processes where the causal connections among events are
represented by graphs. Recently, random walk theory, which addresses a particular class of Markov chain models,
has been applied with some success to the realization of web page ranking algorithms [15], [18]. Internet search
engines use ranking algorithms to measure the relative “importance” of web pages. Such measurements are generally
exploited, along other page features, by “horizontal” search engines, e.g., Google [15], or by personalized search
engines (“vertical” search engines, see, e.g., [19]) to sort the universal resource locators (URLs) returned on user
queries
2
. Some attempts have been made to extend these models with learning capabilities such that a parametric
model representing the behavior of the system can be estimated from training examples [19], [37], [38]. Such
models are able to generalize the results to score all the web pages in the collection.
2
The relative importance measure of a web page is also used to serve other goals, e.g. to improve the efficiency of crawlers [36].
May 24, 2007 DRAFT
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