A necessary and sufficient condition for an identifying independent random process is that
the finite-dimensional probability density function of the process can be expressed with a product of
some one-dimensional functions. The only difference between the one-dimensional functions and the
corresponding marginal density function is a constant, and the boundary constants are irrelevant
with variables and parameters. Those are two important characters of density functions of
independent random processes. Using those characters to avoid solving the marginal density function
makes the identification of an independence of random process simple.