zed vs zeta

zed

noun
  • The name of the Latin-script letter Z. 

  • Something Z-shaped. 

  • Sleep. 

  • A zombie. 

verb
  • To sleep or nap. (Compare zzz, catch some z's.) 

  • To zigzag; to move with sharp alternating turns. 

zeta

noun
  • The sixth letter of the modern Greek alphabet (Ζ (Z), ζ (z)) preceded by epsilon (Ε (E), ε (e)) and followed by eta, (Η (I), η (i)); or the seventh letter in the ancient Greek alphabet, in which it is preceded by digamma (Ϝ (W), ϝ (w)) 

  • A mathematical function formally known as the Riemann zeta function. 

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