 A minimum DRS formula_50 is not necessarily unique, but its dimension is equal to the structural dimension formula_45 of formula_13, by definition.If formula_50 has basis formula_33 and is a minimum DRS, then a plot of "y" versus formula_42 is a minimal sufficient summary plot, and it is ("d" + 1)-dimensional.If a subspace formula_50 is a DRS for formula_13, and if formula_61 for all other DRSs formula_62, then it is a central dimension reduction subspace, or simply a central subspace, and it is denoted by formula_63.