1. The problem asks to write the relation $R1 = \{(x,y) : x < y\}$.\n\n2. This relation consists of all pairs $(x,y)$ where the first element $x$ is strictly less than the second element $y$.\n\n3. In set-builder notation, it is written as $$R1 = \{(x,y) \mid x < y\}.$$\n\n4. This relation defines an ordering where for each pair, the first number is smaller than the second.