Comment on “On a class of exact solutions of the equations of motion of a second grade fluid” by C. Fetecău and J. Zierep (Acta Mech. 150, 135–138, 2001)

In a 2001 article, Fetecau and Zierep considered Stokes' first problem for a second-grade (SG) fluid, unaware that it had already been solved (correctly) by Puri in 1984. These authors used the Fourier sine transform to obtain what they believed to be the exact solution of the resulting initial-boundary-value problem (IBVP). Unfortunately, due to their incorrect computation of the distributional derivative of the Heaviside function, an omission of a critical term in the subsidiary equation occurred, an elementary mistake apparently not uncommon in the literature. Therefore, the solution given is, generally speaking, incorrect. The aim of the present comment is to point out and correct this error.