A note on Riley polynomials of $2$-bridge knots
  In this short note we show the existence of an epimorphism between groups of $2$-bridge knots by means of an elementary argument using the Riley polynomial. As a corollary, we give a classification of $2$-bridge knots by Riley polynomials. 
Teruaki Kitano and Takayuki Morifuji
math.GT