1. A computer-readable medium having computer-executable instructions for controlling a processor of a computer system to categorize a document by a method comprising: for each of a plurality of categories, providing documents within that category, each document having words with characters; for each document, generating a high-order tensor having an order of at least three, each order represented by a coordinate with characters as dimensions of the coordinate, each element of the high-order tensor representing a sequence of at least three characters and being set to a weight based on number of occurrences of that sequence of at least three characters within the document, the weight being based on term frequency by inverse document frequency; and generating a core tensor by reducing dimensionality of the generated high-order tensor using high-order singular value decomposition; training a support vector machine (“SVM”) classifier using the generated core tensors for the documents and the categories of the documents; and categorizing a document by generating a high-order tensor for the document, generating a core tensor for the generated high-order tensor for the document, and applying the SVM classifier to the generated core tensor for the document to determine a category for the document.