  Using previously formulated sum rules in the heavy quark limit of QCD, we
demonstrate that if the slope rho^2 = -xi'(1) of the Isgur-Wise function xi(w)
attains its lower bound 3/4, then all the derivatives (-1)^L xi^(L)(1) attain
their lower bounds (2L+1)!!/2^(2L), obtained by Le Yaouanc et al. This implies
that the IW function is completely determined, given by the function xi(w) =
[2/(w+1)]^(3/2). Since the so-called BPS condition proposed by Uraltsev implies
rho^2 = 3/4, it implies also that the IW function is given by the preceding
expression.
