Regularity of Conjugacies between Critical Circle Maps: An Experimental Study. - Rafael de la Llave; Nikola P. Petrov (Exp. Math., 2002)

Regularity Properties of Critical Invariant Circles of Twist Maps, and Their Universality. - Arturo Olvera; Nikola P. Petrov (SIAM J. Appl. Dyn. Syst., 2008)