doubtless vs without (a) doubt

doubtless

adj
  • Characterized by or experiencing no doubt at all, certain; undoubted; undoubting. 

adv
  • Without doubt; very probably, in all likelihood; doubtlessly. 

without (a) doubt

How often have the words doubtless and without (a) doubt occurred in a corpus of books? (source: Google Ngram Viewer )