 For example, suppose "n" = 3, "k" = 2 and "U" is the triangle formula_14 in formula_15 with vertices "A", "B" and "C" lying on the formula_16-, formula_17- and formula_18-axes, respectively.The subsets formula_19 of formula_20 with exactly 2 elements are formula_21, formula_22 and formula_23.By definition, formula_24 is the orthogonal projection of formula_25 onto the formula_26-plane, so formula_24 is the triangle formula_28 with vertices "O", "B" and "C", where "O" is the origin of formula_15.