If f(x)=∑n=0∞xnn!⋅n!f(x) = \sum_{n=0}^{\infty} \frac{x^n}{n! \cdot n!}f(x)=∑n=0∞n!⋅n!xn, which differential equation does f(x)f(x)f(x) satisfy?
xf′′+f′−f=0xf'' + f' - f = 0xf′′+f′−f=0
x2f′′+xf′−f=0x^2f'' + xf' - f = 0x2f′′+xf′−f=0
f′′−f=0f'' - f = 0f′′−f=0
xf′′+f′+f=0xf'' + f' + f = 0xf′′+f′+f=0