  Using density-functional-based method, we study the k-resolved sigma- and
pi-band holes in Mg_{1-x}Al_{x}B_{2} and Mg(B_{1-y}C_{y})_{2} alloys. We find
that the calculated profiles of the loss of sigma- and pi-band holes in these
two systems as a function of impurity concentration are in qualitative
agreement with experiments, as expected. We also describe its implications
vis-a-vis superconductivity in Mg_{1-x}Al_{x}B_{2} and Mg(B_{1-y}C_{y})_{2}.
