Welcome to the sixth issue in 2021. I am very pleased to announce the journals’ Scopus CiteScore of 2.0 for 2020 indicating another scientifically successful year. On behalf of the J.UCS team, I would like to thank all authors for their sound research contributions, the reviewers for their very helpful suggestions and the consortium members for their financial support. Your commitment and dedicated work have strongly contributed to the long-lasting success of our journal.

In this regular issue, I am very pleased to introduce five accepted papers from six different countries and 17 involved authors.

Edinelço Dalcumune, Luis Antonio Brasil Kowada, André da Cunha Ribeiro, Celina Miraglia Herrera de Figueiredo and Franklin de Lima Marquezino from Brazil present in their article a new algorithm for synthesis of reversible circuits for arbitrary n-bit bijective functions using generalized Toffoli gates, which include positive and negative controls. Murat Firat, Derya Yiltas-Kaplan and Ruya Samli introduce their work on a machine learning method - including Artificial Neural Network (ANN), Linear Regression (LR) and Gradient Boosting (GB) - for determining optimal seat capacity that can supply the highest load factor for the flight operation between any two countries. In a collaborative research between Switzerland, China and the Netherlands Fabian Honegger, Yuan Feng and Matthias Rauterberg have investigated in their research effects of visual, auditory, vibration and draught stimuli on the sense of presence. Julio Moreno, David G. Rosado, Luis E. Sánchez, Manuel A. Serrano and Eduardo Fernández-Medina from Spain discuss in their research a security reference architecture for cyber-physical systems. Adem Tuncer from Turkey introduces a new approach based on a Artificial Bee Colony Algorithm for solving the 15-puzzle problem.