fluctuation vs soundness

fluctuation

noun
  • A wavering; unsteadiness. 

  • In medicine, a wave-like motion or undulation of a fluid in a natural or abnormal cavity (e.g. pus in an abscess), which is felt during palpation or percussion. 

  • A motion like that of waves; a moving in this and that direction. 

soundness

noun
  • The result or product of being sound. 

  • The property (of an argument) of not only being valid, but also of having true premises. 

  • The property of a logical theory that whenever a wff is a theorem then it must also be valid. Symbolically, letting T represent a theory within logic L, this can be represented as the property that whenever T⊢𝜙 is true, then T vDash 𝜙 must also be true, for any wff φ of logic L. 

  • The state or quality of being sound. 

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