Amplitudes of the logarithmic singularities for the four-dimensional critical behavior

In four dimensions mean field theory is not valid since scaling is violated by powers of logarithms. Therefore the theory is not the ε = 0 limit of the 4 - ε dimensional theory. However ratios of critical amplitudes are in most cases correctly described by this limit. In some cases, it is nevertheless necessary to be cautious, and an example is provided of a ratio which does not coincide with its mean field value.