IMO 1983 LL CAN17
In how many ways can 1, 2, . . . , 2n be arranged in a 2 imes n
IMO 1983 LL CAN17
Origin: CAN
Problem
In how many ways can 1, 2, . . . , 2n be arranged in a 2 \times n rectangular array a1 a2 \cdot \cdot \cdot an b1 b2 \cdot \cdot \cdot bn for which: (i) a1 < a2 < \cdot \cdot \cdot < an, (ii) b1 < b2 < \cdot \cdot \cdot < bn, (iii) a1 < b1, a2 < b2, . . . , an < bn?