Solution of Bessel's equation

Question

I am trying to solve Bessel's equation ${x^2}{y''}+{x}{y'}+{\left(x^2-v^2\right)}{y}=0$ . When I reached $c_{2n}=\frac{\left(-1\right)^nc_0}{2^{2n} n!\left(1+v\right)\left(2+v\right)\ldots\left(n+v\right)}$ then in zill book it is written that "it is standard practice to choose $c_0$ to be specific value , namely $c_0=\frac{1}{2^v\Gamma\left(1+v\right)}$" my question how $c_0$ is choosen. Elaborate it