<a href="https://doi.org/10.53656/math2021-4-8-res" target="_blank" rel="noopener noreferrer">https://doi.org/10.53656/math2021-4-8-res</a> <a href="https://azbuki.bg/wp-content/uploads/2021/08/Resheniya.pdf" target="_blank" rel="noopener noreferrer">Отвори пълния текст</a>