mu vs Μ variant

mu

intj
  • Neither yes nor no. 

noun
  • The 12th letter of the Modern Greek alphabet. 

  • A unit of surface area, currently equivalent two-thirds of a decare (about 666 meters squared). 

  • Nothingness; nonexistence; the illusory nature of reality. 

Μ variant

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