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# Determine the interval of convergence of ∑ xn / n! and show that it satisfies y′ = x + y

Consider the function defined by the power series Determine the interval of convergence for and show that it satisfies the differential equation (We might notice that this is almost the power series expansion for the exponential function and deduce the interval of convergence and the differential equation from properties of the exponential that we already know. We can do it from scratch just as easily though.)

First, we apply the ratio test Hence, the series converges for all . Next, we take a derivative Then we have Now, to compute the sum we can solve the given differential equation This is a first order linear differential equation of the form with and . We also know that ; therefore, this equation has a unique solution satisfying the given initial condition which is given by Where and Therefore, we have 