Frictionless and inviscid In the book Fluid Mechanics by  Robert A. Granger, there is a study question, 4.10, asking 
"How can frictionless real fluids exist and inviscid fluids not exist?"
Could someone please explain?
Please note that this is no homework-question, but is part of a self-study. I have tried to google around, but could not find any answers.  I have seen several webpages stating that frictionless is the same as inviscid, for instance 
http://wwwcourses.sens.buffalo.edu/mae335/files/assignment/Lecture_Notes_11_03.pdf
 A: I think this is a pretty ridiculous question (not yours, the textbook authors'). But my guess would be that the distinction is made between inviscid fluids which are governed by the Euler equations:
$$\frac{\partial u_i}{\partial t} + \frac{\partial u_i u_j}{\partial x_i} = -\frac{1}{\rho} \frac{\partial p}{\partial x_i}$$
and frictionless fluids which are governed by the Navier-Stokes equations:
$$\frac{\partial u_i}{\partial t} + \frac{\partial u_i u_j}{\partial x_i} = -\frac{1}{\rho}\frac{\partial p}{\partial x_i} + \nu \frac{\partial^2 u_i}{\partial x_i \partial x_j}$$ 
where $\nu = 0$. 
In other words, inviscid is a mathematical assumption where we assume the viscous term does not exist in the equation at all (which changes the mathematical nature of the equation) and frictionless is a physical property of the fluid where the viscosity is zero (which has the exact same effect as being inviscid). 
Poor question, but I think they want you to understand the difference between mathematical assumptions and physical properties.
