COCI, the OpenCitations Index of Crossref open DOI-to-DOI citations

Open Citation Identifiers Each Open Citation Identifier [[OCI]] has a simple structure: the lower-case letters "oci" followed by a colon, followed by two numbers separated by a dash (e.g. https://w3id.org/oc/index/coci/ci/02001010806360107050663080702026306630509-02001010806360107050663080702026305630301), in which the first number identifies the citing work and the second number identifies the cited work. For citations in which the citing and cited works are identified by DOIs, which includes all the COCI citations, the OCI is created in the following manner, as explained more fully here. Each case-insensitive DOI is first normalized to lower case letters. Then, after omitting the initial doi:10. prefix, the alphanumeric string of the DOI is converted reversibly to a pure numerical string using the simple two-numeral lookup table for numerals, lower case letters and other characters presented at https://github.com/opencitations/oci/blob/master/lookup.csv. Finally, each converted numeral is prefixes by a 020, which indicates that Crossref is the supplier of the original metadata of the citation (as indicated at http://opencitations.net/oci). OCIs can be resolved using the OpenCitations OCI Resolution Service.