UCNC 2018 special issue editorial

Natural computing refers to the interdisciplinary area between computer science, mathematics, physics and biology that tries to formalise and explore the computational processes found in the physical world. It aims to copy natural processes in order to provide new solutions to computationally hard problems. Another investigated direction is the implementation of computation in (nonelectronic) physical media. It is not surprising that in many cases the computation performed by obtained formalisms does not fit into the classical concept of Turing computation, mainly because of the highly distributed and parallel structure. The area of unconventional computation studies computational models that do not fit into the classical modes of computation, including models not having a physical inspiration. The research in these aforementioned domains is extremely challenging as many concepts are completely new and were never considered before. This causes many technical difficulties, in particular for formalisation and understanding of the computational process. At the same time the study of such models is intellectually rewarding, providing deep unsuspected connections between areas of mathematics, physics, biology and computer science. The Unconventional Computation and Natural Computation (UCNC) conference is the main venue for researchers in these fields. The first UCNC (at that time called Unconventional Models of Computation) was held in Auckland, New Zealand, in 1998, organised by the Centre for Discrete Mathematics and Theoretical Computer Science, University of Auckland, and the Santa Fe Institute. Since then, it has been held in Brussels, Belgium (2000), Kobe, Japan (2002), Seville, Spain (2005), York, UK (2006), Kingston, Canada (2007), Vienna, Austria (2008), Ponta Delgada, Azores, Portugal (2009), Tokyo, Japan (2010), Turku, Finland (2011), Orléans, France (2012), Milano, Italy (2013), London, Ontario, Canada (2014), Auckland, New Zealand (2015), Manchester, UK (2016), Fayetteville, USA (2017), Fontainebleau, France (2018) and Tokyo, Japan (2019). The 17th edition of UCNC was held June 25–29, 2018, on the campus of the IUT de Fontainebleau at the University of Paris-Est Créteil – Val de Marne, Fontainebleau, France. The meeting was pleased to have four distinguished invited speakers who presented inspiring talks related to several UCNC topics: Satoshi Murata (Tohoku University, Japan), Julian Miller (University of York, UK), Lee Cronin (University of Glasgow, UK) and Damien Woods (INRIA, France). Also the meeting featured a tutorial provided by Jean Krivine (CNRS and University of Paris Didérot, France). The meeting was accompanied by four associated workshops and co-located with Machines, Computations and Universality (MCU 2018) conference. This special issue of Natural Computing contains a selection of papers from UCNC 2018, which underwent substantial updates and extensions, and went through an independent review process. The paper ‘‘The alchemy of computation: designing with the unknown’’ by Julian Miller is the full version of his invited talk presented at the conference. It presents and compares two computer controlled search approaches for computational problems solving: Cartesian genetic programming and the evolution in materio. The paper also discusses possible new research directions by borrowing ideas from one and using them in the other. The paper ‘‘Optimal Staged Self-Assembly of Linear Assemblies’’ by Cameron Chalk, Eric Martinez, Robert Schweller, Luis Vega, Andrew Winslow and Tim Wylie analyses the complexity of building linear assemblies, sets of linear assemblies, and O(1)-scale general shapes in the staged tile assembly model. Upper and lower bounds for the minimal number of stages needed to perform & Susan Stepney susan.stepney@york.ac.uk