1∕F as a functionof r_i. In addition to the trivial case q = 0 (dotted line), four representative cases are considered. Dashed and solid lines respectively correspond to q < 0 and q > 0, while blue indicates curves with a + q > 0 and a > 0 and red in all the other cases. The blue dots indicate the value ((a + q)∕a) of the corresponding curve for r_i → 0⁺ and the black dot indicates the limit (1) of all curves for r_i → 1⁻.