The exact region determined by Blomqvist’s beta, Spearman’s footrule and Gini’s gamma

To quantify the degree of association between random variables, concordance measures are employed. To express such a degree, a single measure might give too much space, so several are used for comparison. In this paper we study the ternary relation between three well-known (weak) concordance measures, namely Blomqvist’s beta, Spearman’s footrule and Gini’s gamma. In other words, given the values of Blomqvist’s beta and Spearman’s footrule, we determine the degree of freedom a copula has at taking the value of Gini’s gamma. We explicitly determine the 3-dimensional region representing the relation. We also provide copulas where bounds of the region are attained.