Xi vs si

Xi

noun
  • Capital Greek letter Ξ (X) 

name
  • A river in China, the western tributary of the Pearl. 

  • A county of Linfen, Shanxi, China. 

  • Xi Jinping, current General Secretary of the Chinese Communist Party (2012–present). 

  • A surname from Chinese 

si

noun
  • A syllable used in solfège to represent the seventh note of a major scale. 

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