  We cannot use directly the results of zero-temperature at finite temperature,
for at finite temperature the average is to be carried over all highly
degenerate excited states unlike zero-temperature average is only on unique
ground state. One of the formal way to take into account the finite temperature
into quantum field theory is due to Matsubara, to replace temporal component of
eigenvalues $k_{4}$ by $\omega_{n}=\frac{2\pi n}{\beta}$ $(\frac{2\pi
(n+{1/2})}{\beta})$ with summation over all integer values of $n$. The
summation is done with the infinite series expansion of $\coth (\pi y)$. With
the chemical potential $\mu$, $\omega_{n}$ will be replaced by $\omega_{n} -
\mu$ in the eigenvalues and the summation over $n$ cannot be done easily.
Various methods exist to evaluate it. We use the infinite series expansion of
$\coth (\pi y)$ to work operationally for such Matsubara frequency sums.
