invariant vs unfailing

invariant

adj

  • not varying; constant 

  • Neither covariant nor contravariant. 

  • Unaffected by a specified operation (especially by a transformation) 

noun

  • An invariant quantity, function etc. 

unfailing

adj

  • changeless, constant 

  • inexhaustible 

  • infallible 

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