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Multivariate distributions with fixed marginals and correlations

Consider the problem of drawing random variates $(X_1,\ldots,X_n)$ from a distribution where the marginal of each $X_i$ is specified, as well as the correlation between every pair $X_i$ and $X_j$. For given marginals, the Fréchet-Hoeffding bounds put a lower and upper bound on the correlation between $X_i$ and $X_j$. Any achievable correlation between $X_i$ and $X_j$ is a convex combinations of these bounds. The value $λ(X_i,X_j) \in [0,1]$ of this convex combination is called here the convexity parameter of $(X_i,X_j),$ with $λ(X_i,X_j) = 1$ corresponding to the upper bound and maximal correlation. For given marginal distributions functions $F_1,\ldots,F_n$ of $(X_1,\ldots,X_n)$ we show that $λ(X_i,X_j) = λ_{ij}$ if and only if there exist symmetric Bernoulli random variables $(B_1,\ldots,B_n)$ (that is $\{0,1\}$ random variables with mean 1/2) such that $λ(B_i,B_j) = λ_{ij}$. In addition, we characterize completely the set of convexity parameters for symmetric Bernoulli marginals in two, three and four dimensions.