Comments on the paper by M. F. Letelier and D. A. Siginer : On the flow of a class of viscoinelastic-viscoplastic fluids in tubes of non-circular contour. International Journal of Engineering Science, Vol. 45 (2007), 873-881

It is shown that the solution to the steady flow of a Newtonian fluid in a pipe of equilateral triangular cross-section is wrong, for it is based on an inappropriate choice of a parameter and the incorrect use of trigonometry. Secondly, the assumption that the shear stresses in a steady flow in a pipe are given by the gradient of a scalar function is also incorrect, since it depends on the equality between directional and partial derivatives everywhere.