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<article xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:ali="http://www.niso.org/schemas/ali/1.0/" article-type="research-article" dtd-version="1.2" xml:lang="en"><front><journal-meta><journal-id journal-id-type="publisher-id">Contemporary Mathematics. Fundamental Directions</journal-id><journal-title-group><journal-title xml:lang="en">Contemporary Mathematics. Fundamental Directions</journal-title><trans-title-group xml:lang="ru"><trans-title>Современная математика. Фундаментальные направления</trans-title></trans-title-group></journal-title-group><issn publication-format="print">2413-3639</issn><issn publication-format="electronic">2949-0618</issn><publisher><publisher-name xml:lang="en">Peoples’ Friendship University of Russia named after Patrice Lumumba (RUDN University)</publisher-name></publisher></journal-meta><article-meta><article-id pub-id-type="publisher-id">34591</article-id><article-id pub-id-type="doi">10.22363/2413-3639-2023-69-1-18-31</article-id><article-id pub-id-type="edn">CNBPXH</article-id><article-categories><subj-group subj-group-type="toc-heading" xml:lang="en"><subject>Articles</subject></subj-group><subj-group subj-group-type="toc-heading" xml:lang="ru"><subject>Статьи</subject></subj-group><subj-group subj-group-type="article-type"><subject>Research Article</subject></subj-group></article-categories><title-group><article-title xml:lang="en">Quasilinear elliptic and parabolic systems with nondiagonal principal matrices and strong nonlinearities in the gradient. Solvability and regularity problems</article-title><trans-title-group xml:lang="ru"><trans-title>Квазилинейные эллиптические и параболические системы с недиагональными главными матрицами и сильными нелинейностями по градиенту. Проблемы разрешимости и регулярности</trans-title></trans-title-group></title-group><contrib-group><contrib contrib-type="author"><name-alternatives><name xml:lang="en"><surname>Arkhipova</surname><given-names>A. A.</given-names></name><name xml:lang="ru"><surname>Архипова</surname><given-names>Арина Алексеевна</given-names></name></name-alternatives><email>arinaark@gmail.com</email><xref ref-type="aff" rid="aff1"/></contrib></contrib-group><aff-alternatives id="aff1"><aff><institution xml:lang="en">St. Petersburg State University</institution></aff><aff><institution xml:lang="ru">Санкт-Петербургский государственный университет</institution></aff></aff-alternatives><pub-date date-type="pub" iso-8601-date="2023-03-31" publication-format="electronic"><day>31</day><month>03</month><year>2023</year></pub-date><volume>69</volume><issue>1</issue><issue-title xml:lang="en">Differential and Functional Differential Equations</issue-title><issue-title xml:lang="ru">Дифференциальные и функционально-дифференциальные уравнения</issue-title><fpage>18</fpage><lpage>31</lpage><history><date date-type="received" iso-8601-date="2023-05-05"><day>05</day><month>05</month><year>2023</year></date></history><permissions><copyright-statement xml:lang="en">Copyright ©; 2023, Arkhipova A.A.</copyright-statement><copyright-statement xml:lang="ru">Copyright ©; 2023, Архипова А.А.</copyright-statement><copyright-year>2023</copyright-year><copyright-holder xml:lang="en">Arkhipova A.A.</copyright-holder><copyright-holder xml:lang="ru">Архипова А.А.</copyright-holder><ali:free_to_read xmlns:ali="http://www.niso.org/schemas/ali/1.0/"/><license><ali:license_ref xmlns:ali="http://www.niso.org/schemas/ali/1.0/">https://creativecommons.org/licenses/by-nc/4.0</ali:license_ref></license></permissions><self-uri xlink:href="https://journals.rudn.ru/CMFD/article/view/34591">https://journals.rudn.ru/CMFD/article/view/34591</self-uri><abstract xml:lang="en"><p style="text-align: justify;">We consider nondiagonal elliptic and parabolic systems of equations with strongly nonlinear terms in the gradient. We review and comment known solvability and regularity results and describe the last author’s results in this field.</p></abstract><trans-abstract xml:lang="ru"><p style="text-align: justify;">Изучаются недиагональные эллиптические и параболические системы уравнений с сильно нелинейными членами по градиенту. Мы рассматриваем и комментируем известные результаты о разрешимости и регулярности и описываем последние результаты автора в этой области.</p></trans-abstract><kwd-group xml:lang="en"><kwd>nonlinear elliptic systems</kwd><kwd>nonlinear parabolic systems</kwd><kwd>regularity of weak solutions</kwd></kwd-group><kwd-group xml:lang="ru"><kwd>нелинейные эллиптические системы</kwd><kwd>нелинейные параболические системы</kwd><kwd>регулярность слабых решений</kwd></kwd-group><funding-group/></article-meta></front><body></body><back><ref-list><ref id="B1"><label>1.</label><mixed-citation>Архипова А. А. 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