E.A. Grove & G. Ladas 
Periodicities in Nonlinear Difference Equations [PDF ebook] 

Sharkovsky’s Theorem, Li and Yorke’s "period three implies chaos" result, and the (3x+1) conjecture are beautiful and deep results that demonstrate the rich periodic character of first-order, nonlinear difference equations. To date, however, we still know surprisingly little about higher-order nonlinear difference equations. During the last