From the text: We are honored to dedicate this special issue to the memory of Ker-I Ko, a colleague and a long-time friend, for his many contributions to the research in theoretical computer science. Ko passed away on the 13th of December in 2018. Various commemorations have been held to celebrate his achievements, including an ad hoc International Workshop on Complexity and Approximation that took place in Qingdao in April 2019 and a special volume of collected works published by Springer in 2020 (LNCS Vol. 12000 [Zbl 1435.68027]). This special issue adds to these activities from a different perspective. In particular, this issue includes a paper titled “On continuous one-way functions” [K.-I Ko and L. Wu, Theor. Comput. Sci. 852, 1–17 (2021; Zbl 1498.03096)] based on Ko’s manuscript.