Proceedings of the Royal Society of Edinburgh. Section A. Mathematics (Proc. Roy. Soc. Edinburgh Sect. A) (19810101), 91, no.~1-2, 101-105. ISSN: 0308-2105 (print).eISSN: 1473-7124.
Institute of Electrical and Electronics Engineers. Transactions on Automatic Control (IEEE Trans. Automat. Control) (19810101), 26, no.~5, 988-993. ISSN: 0018-9286 (print).eISSN: 1558-2523.