Past exam of the mathematics course of the University of Cambridge / 2002 / iii / Paper 23 / 10 / a / Solution

Use only the variable names $x,y$. Define mutually renamed formulas bywhere $D_m^y$ is obtained from $D_m^x$ by globally interchanging $x$ and $y$, including their bound occurrences. By induction, $D_m^x(a)$ means that an increasing chain of $m$ elements starts at $a$. Henceholds exactly when the linear order has at least $n$ elements. Inner quantifiers may bind the name $x$ again, but their scope is only the nested subformula; the comparison $x<y$ still refers to the preceding point. This finite-variable logic construction works on finite and infinite linear orders.