Luong, D.-T., Vu Thi, T.-L. and Dang, T.-N. 2025. A Lagrange function approach to study second-order optimality conditions for infinite-dimensional optimization problems. HPU2 Journal of Science: Natural Sciences and Technology. 4, 01 (Apr. 2025), 20–30. DOI:https://doi.org/10.56764/hpu2.jos.2025.4.01.20-30.