My questions concerns the information in the document. I highlighted the portion that is confusing me and a sample problem at the bottom. Question: Look at the equation 2.2.4 in the document. When I set the function u equal to zero the equation becomes 0 = 0 + 0 + f(x,t) or f(x,t) = 0. Now if you look in the document once more at the underlined green section is says that if f is equal to zero then the equation is homogeneous. However if you read the underlined orange section it says equation 2.2.4 is a nonhomogeneous equation...... This seems a bit contradictory. Some one please help me understand what they are saying.