Before revision: It is well-known that an \R^n-valued random vector (X_1, X_2, \cdots, X_n) is comonotonic if and only if (X_1, X_2, \cdots, X_n) and (Q_1(U), Q_2(U),\cdots, Q_n(U)) coincide in distribution, for any uniformly distributed random variable U  on the unit interval , where Q_k is the quantile function of X_k, k=1,2,\cdots, n. It is natural to ask whether (X_1, X_2, \cdots, X_n) and (Q_1(U), Q_2(U),\cdots, Q_n(U)) can coincide almost surely for some special U.
After revision: It is well-known that an \R^n-valued random vector (X_1, X_2, \cdots, X_n) is comonotonic if and only if (X_1, X_2, \cdots, X_n) and (Q_1(U), Q_2(U),\cdots, Q_n(U)) coincide in distribution, for any uniformly distributed random variable U on the unit interval , where Q_k (\cdot) are the quantile functions of X_k, k=1,2,\cdots, n. It is natural to ask whether (X_1, X_2, \cdots, X_n) and (Q_1(U), Q_2(U),\cdots, Q_n(U)) can coincide almost surely for some special U.