In this paper we show that the solution of the Stokes problem has square integrable second derivatives provided that the domain is a convex polygon in the plane and the nonhomogeneous term is square integrable. © 1976.
Kellogg, R. B., & Osborn, J. E. (1976). A regularity result for the Stokes problem in a convex polygon. Journal of Functional Analysis, 21(4), 397–431. https://doi.org/10.1016/0022-1236(76)90035-5