Solution
= Solution
The universal <stress-tensor operator-product expansion> in a <two-dimensional conformal field theory> is
$$
T(z)T(w)\sim
\frac{c/2}{(z-w)^4}
+\frac{2T(w)}{(z-w)^2}
+\frac{\partial T(w)}{z-w}.
$$
Its fourth-order coefficient defines the <central charge> $c$.