an vs within

an

prep
  • In each; to or for each; per. 

noun
  • The first letter of the Georgian alphabet, ა (Mkhedruli), Ⴀ (Asomtavruli) or ⴀ (Nuskhuri). 

article
  • Used before one and words with initial u, eu. 

  • Used before a vowel sound. 

  • Used before all consonants. 

  • Used before /h/ in an unstressed syllable. 

  • Used before /h/ in a stressed syllable. 

num
  • one 

within

prep
  • In the scope or range of. 

  • Before the specified duration ends. 

  • In the inner part, spatially; physically inside. 

adj
  • In the context of which the present document or ruling is made. 

adv
  • In or into the interior; inside. 

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