complexity vs impenetrability

complexity

noun
  • The state of being complex; intricacy; entanglement. 

  • That which is and renders complex; intricacy; complication. 

impenetrability

noun
  • The characteristic of being impenetrable; invulnerability. 

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