Repositorio Institucional de la Universidad de Oviedo

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The authors contributed equally at all stages of the preparation of the paper. Sections 2-4 were written by the first author who was supported by the RSF grant no. 19-11-00069, Russia, and by the Portuguese Foundation for Science and Technology, Portugal, under the project UID/MAT/04561/2019. Section 1 was written by the second author supported by the Ministry of Science and Higher Education of the Russian Federation under project no. III.22.4.2, Russia. Sections 5-7 were prepared by the third author who acknowledges the support of the Ministry of Economy, Industry, and Competitiveness grant no. MTM2017- 87162-P, Spain.