Author’s summary: “We study the regularity on the lateral boundary of where u is a weak solution of the mixed problem for the equation
is a bounded open set of with regular boundary and . Here and are real functions such that and , for -, appropriately chosen. This type of result was called by J. L. Lions [Mat. Apl. Comput. 6, 7-16 (1987; Zbl 0656.35097)] of hidden regularity of u.