1. A computerized search engine for identifying and ranking relevant documents from a corpus of citationally-related documents, said computerized search engine comprising: an input interface that enables a user to select a first set of identification information identifying one or more input documents from said corpus of citationally-related documents; a computer-accessible index stored in a computer-readable storage device, said computer-accessible index comprising identification information identifying each potential input document from said corpus of citationally-related documents and, for each said potential input document, identification information identifying a selected number of citationally-related potential output documents from said corpus of citationally-related documents, said computer-accessible index further comprising for each pair of citationally-related potential input document and potential output document a first numerical score that is statistically correlated to the probability that a direct citation exists between each said pair of citationally related documents and wherein said first numerical score is calculated based at least in part on how many indirect citations exist between each said pair of citationally related documents and, for each indirect citation, how many citation links separate each said pair of citationally-related documents; a computer processor configured to execute instructions stored in a computer-readable storage device, said instructions configured to cause said computer processor to: access, from said computer-accessible index, a second set of identification information identifying one or more output documents corresponding to each of said one or more input documents and, for each identified pair of citationally-related input document and output document, said corresponding first numerical score; and calculate, for each identified output document, a second numerical score that is statistically correlated to the probability that a direct citation exists between any of said one or more input documents and each said identified output document, and wherein said second numerical score is calculated based at least in part on said first numerical score; and an output interface to display search results comprising identification information corresponding to said one or more output documents and wherein said search results are ranked in accordance with said second numerical score.