Trapezoidal Chains and Antichains

Further common properties of the partially ordered sets R(m, n)={(i, j) ∈ ℤ2: 1 ⩽ i ⩽ m, 1 ⩽ j ⩽ n} (a rectangle) and T(m, n) = {(i, j)∈ ℤ2: 1 ⩽ i ⩽ j ⩽ m + n - 1} (a trapezoid) are established. Weights are attached to the antichains of R(m, n) and T(m, n)in such a way that the sum of the weights of the antichains in R(m, n) and T(m, n) are the same. A proof is given which exploits connections among antichains in T(m, n), the theory of Schur functions, and self-complementary tableaux. A bijection is given between the antichains of R (m, n) and T(m, n) which uses jeu de taquin. Similar weights, theorems, and bijections are also given for the multichains of R(m, n) and T(m, n).