Past exam of the year 1 of the computer science course of the University of Oxford / 2023 / Discrete mathematics, probability and continuous mathematics / 1 / b / i / Solution

For the product order, reflexivity follows from $a_i\le a_i$ in every coordinate. If $\mathbf{a}⪯\mathbf{b}$ and $\mathbf{b}⪯\mathbf{a}$, the antisymmetric relation axiom for the usual real order gives $a_i=b_i$ for every $i$, hence $\mathbf{a}=\mathbf{b}$. If $\mathbf{a}⪯\mathbf{b}⪯\mathbf{c}$, transitivity in each coordinate gives $a_i\le c_i$, hence $\mathbf{a}⪯\mathbf{c}$. Thus this is a partial order.

For $n\ge 2$, the vectors $(1,0,0,\dots )$ and $(0,1,0,\dots )$ are incomparable: each has a coordinate larger than the corresponding coordinate of the other. It is not a total order for $n\ge 2$. The dimension qualification matters: for $n=1$ this is the usual total order on $\mathbb{R}$, so the printed non-totality claim needs the implicit assumption $n\ge 2$.