Special issue on parametric excitation: applications in science and engineering

There are many reasons for offering a Special Issue on parametrically excited systems at this point in time. Here are the main motivators: (a) it is clearly of wide- spread interest to demonstrate the diversity of the ori- gins of such systems on the one hand, and on the other to recognise the fact they are all, in general, defined by the existence of time-varying coefficients in the govern- ing equation motion; (b) parametrically excited systems are characterised by particularly distinctive behaviour and phenomena; (c) they are associated with a vast number of applications in science and engineering, with emerging fields expanding through the rediscovery of the benefits of introducing intentional time-periodicities into certain systems of interest; and (d) last but not least – this current time can be seen as an anniversary – the article that represents a milestone for their investigation was published about 180 years ago by Faraday. 1,2 Thus, before immersing ourselves in the research collected in this Special Issue, it is logical and natural to look back to those pioneered researchers who set the scene at the end of the 19th-century and in the early years of the 20th- century.