correspondence vs equation

correspondence

noun
  • An agreement of situations or objects with an expected outcome. 

  • A relation. 

  • Reciprocal exchange of civilities, especially conversation between persons by means of letters. 

  • Postal or other written communications. 

  • Newspaper or news stories. 

  • A similarity between physical and spiritual things (e.g. light to wisdom, or warmth to love) 

equation

noun
  • The act or process of equating two or more things, or the state of those things being equal (that is, identical). 

  • An assertion that two expressions are equal, expressed by writing the two expressions separated by an equal sign; from which one is to determine a particular quantity. 

  • A small correction to observed values to remove the effects of systematic errors in an observation. 

How often have the words correspondence and equation occurred in a corpus of books? (source: Google Ngram Viewer )