2. The method of claim 1 , wherein the degree of semantic similarity of each of the plurality of passages from the external documents with respect to the sentence of the document is based on the entirety of the sentence.
1. A method comprising: accessing, at one or more computing devices, a document; identifying a plurality of sentences of the document, each sentence identified based on punctuation in the document; and executing, for each of the sentences of the document and using the one or more computing devices, a document modification operation that includes: generating a ranking score for each of a plurality of passages from external documents, wherein the ranking score is based at least on a degree of semantic similarity of each of the plurality of passages from the external documents with respect to the sentence of the document, modifying the sentence to include a footnote link for the sentence in the document, the footnote link including a link to the external document having a highest ranked passage therein if the ranking score of the highest ranked passage with respect to the sentence exceeds a threshold value, and skipping modification of the sentence if the ranking score of the highest ranked passage with respect to the sentence does not exceed the threshold value.