An elementary introduction to the 2d/4d correspondences is given. After quickly reviewing the 2d q-deformed Yang-Mills theory and the Liouville theory, we will introduce 4d theories obtained by coupling trifundamentals to SU(2) gauge fields. We will then see concretely that the supersymmetric partition function of these theories on S3 × S1 and on S4 is given respectively by the q-deformed Yang-Mills theory and the Liouville theory. After giving a short discussion on how this correspondence may be understood from the viewpoint of the 6d N=(2, 0) theory, we conclude the review by enumerating future directions. Most of the technical points will be referred to more detailed review articles. This is a part of the combined reviews on supersymmetric localization, edited by V. Pestun and M. Zabzine.

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