Detailed numerical simulations of the IV characteristics of a Josephson junction under external electromagnetic radiation show the devil's staircase within different bias current intervals. We have found that the observed steps form very precisely continued fractions. Increase of the amplitude of the radiation shifts the devil's staircase to higher Shapiro steps. An algorithm for the appearance and detection of subharmonics with increasing radiation amplitude is proposed. We demonstrate that the subharmonic steps registered in the well-known experiments by Dayem and Wiegand [Phys. Rev. 155, 419 (1967)] and Clarke [Phys. Rev. B 4, 2963 (1971)] also form continued fractions.

• Received 22 August 2013
• Revised 17 October 2013

DOI:https://doi.org/10.1103/PhysRevB.88.214515