1. Perth College, Univ. of the Highlands and Islands, Crieff Rd., Perth PH1 2NX, UK.
2. Lecturer in Mathematics, School of Engineering and the Built Environment, Edinburgh Napier Univ., Edinburgh EH10 5DT, UK; Perth College, Univ. of the Highlands and Islands, Crieff Rd., Perth PH1 2NX, UK (corresponding author). ORCID: .