tenacious vs unflagging

tenacious

adj
  • Having a good memory; retentive. 

  • Unwilling to yield or give up; dogged. 

  • Holding together; cohesive. 

  • Clinging to an object or surface; adhesive. 

unflagging

adj
  • Never tiring or lacking energy; without rest; without slowing. 

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