  The lower-critical dimension for the existence of the Ising spin-glass phase
is calculated, numerically exactly, as $d_L = 2.520$ for a family of
hierarchical lattices, from an essentially exact (correlation coefficent $R^2 =
0.999999$) near-linear fit to 23 different diminishing fractional dimensions.
To obtain this result, the phase transition temperature between the disordered
and spin-glass phases, the corresponding critical exponent $y_T$, and the
runaway exponent $y_R$ of the spin-glass phase are calculated for consecutive
hierarchical lattices as dimension is lowered.
