k := 1.3807 * 10^(-23) * J / K:
c := 2.9979 * 10^8 * m/s:
hbar := 1.0546 * 10^(-34) * J * s:
bolt := k^4 / (4 * Pi^2 * c^2 * hbar^3) * int(n^3 / (exp(n) - 1), n = 0..infinity):
evalf(bolt);
              -7    J
.5670850646 10   -------
                  4  2
                 K  m  s