“An Example of a Finite Number of Recursively Enumerable $m$-Degrees Containing an Infinite Sequence of Recursively Enumerable Sets”. Annual of Sofia University St. Kliment Ohridski. Faculty of Mathematics and Informatics, vol. 87, Dec. 1995, pp. 223-34, https://annual.uni-sofia.bg/index.php/fmi/article/view/415.