let in vs undergo

let in

verb
  • To associate with. 

  • To let someone or something come in; to admit someone or something in. 

  • To divulge one's inner thoughts to (someone), making oneself emotionally vulnerable to them; to open up to (someone). 

undergo

verb
  • To suffer or endure; bear with. 

  • To experience; to pass through a phase. 

  • The project is undergoing great changes. 

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