  In this paper, we present COCI, the OpenCitations Index of Crossref open
DOI-to-DOI citations (http://opencitations.net/index/coci). COCI is the first
open citation index created by OpenCitations, in which we have applied the
concept of citations as first-class data entities, and it contains more than
445 million DOI-to-DOI citation links derived from the data available in
Crossref. These citations are described in RDF by means of the newly extended
version of the OpenCitations Data Model (OCDM). We introduce the workflow we
have developed for creating these data, and also show the additional services
that facilitate the access to and querying of these data via different access
points: a SPARQL endpoint, a REST API, bulk downloads, Web interfaces, and
direct access to the citations via HTTP content negotiation. Finally, we
present statistics regarding the use of COCI citation data, and we introduce
several projects that have already started to use COCI data for different
purposes.
