co-occurrence vs coöccurrence

co-occurrence

noun

  • The fact of a thing occurring simultaneously with something else; correlation. 

  • An instance of a thing occurring simultaneously with something else; co-incidence. 

coöccurrence

noun

  • The fact of a thing occurring simultaneously with something else; correlation. 

  • An instance of a thing occurring simultaneously with something else; coïncidence. 

How often have the words co-occurrence and coöccurrence occurred in a corpus of books? (source: Google Ngram Viewer )