1. The problem statement, all variables and given/known data Which of the signals is not the result of fourier series expansion? options : (a) 2cos(t) + 3 cos(3t) (b) 2cos([itex]\pi[/itex]t) + 7cos(t) (c) cos(t) + 0.5 2. Relevant equations Dirichlet conditions 3. The attempt at a solution From observation, I thought all are periodic and so must be fourier series expansions. But since this was a question in an objective exam, I went for option (b). Although the answer is right, I am not satisfied, by the explanation of Dirichlet conditions. And the plot of the function 2cos([itex]\pi[/itex]t) + 7cos(t) seems periodic too and no discontinuities etc. My question is actually is 2cos([itex]\pi[/itex]t) + 7cos(t) a function which cannot be a result of fourier expansion or not?