  We investigate numerically and analytically the formation of the frozen core
in critical random Boolean networks with biased functions. We demonstrate that
a previously used efficient algorithm for obtaining the frozen core, which
starts from the nodes with constant functions, fails when the number of inputs
per node exceeds 4. We present computer simulation data for the process of
formation of the frozen core and its robustness, and we show that several
important features of the data can be derived by using a mean-field
calculation.
