be up to vs undertake

be up to

verb
  • To do or be involved in doing; to do (often something mischievous or inappropriate). 

  • To depend on, to be the choice of. 

undertake

verb
  • To take upon oneself; to start, to embark on (a specific task etc.). 

  • To commit oneself (to an obligation, activity etc.). 

  • To pass a slower moving vehicle on the curbside rather than on the side closest to oncoming traffic. 

noun
  • The passing of slower traffic on the curbside rather than on the side closest to oncoming traffic. 

How often have the words be up to and undertake occurred in a corpus of books? (source: Google Ngram Viewer )