1. "The problem statement, all variables and given/known data Find a recurrence formula for the power series solution around x = 0 for the differential equation given in the previous problem." The previous problem says: "Determine whether x = 0 is an ordinary point of the differential equation y'' + y = 0." 2. Relevant equations Power series and related stuff. 3. The attempt at a solution I have the solutions for both of these problems and I also know how to do them both. My question is just: If x = 0 was not an ordinary point, what would that mean? Would that mean that I cannot assume a power series solution of the form y = [n=0 to inf] Σ[a_n (x - x_0)^n] (where x_0 = 0 in this case) exists or what? Any input would be greatly appreciated! Thanks in advance!