invariable vs monotonic

invariable

adj
  • Not variable; unalterable; uniform; always having the same value. 

  • Constant. 

  • That cannot undergo inflection, conjugation or declension. 

noun
  • Something that does not vary; a constant. 

monotonic

adj
  • said of a function that either never decreases or never increases as its independent variable increases. 

  • Uttered in a monotone; monotonous. 

  • of or using the Greek system of diacritics which discards the breathings and employs a single accent to indicate stress. It replaced polytonic system in 1982. 

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