We study Hamamoto's expanding square argument towards a 1-D minimisation problem related to the sharp solenoidal uncertainty principle. Working in the right function space, we recast the involved interpolation type inequality into an exact equality, where the vanishing of the remainder term characterises the extremisers via the confluent hypergeometric functions. In the process we also remove some unnecessary constraints on the prescribed parameters.
Comment: Submitted to a journal on December 26, 2024