Past exam of the mathematics course of the University of Cambridge / 2026 / iii / Paper 130 / 2

For a single homogeneous equationwith nonzero integer coefficients, Rado theorem says that it is a partition regular equation exactly whenfor some nonempty $I\subseteq \{1,\dots ,r\}$.

For necessity, suppose no nonempty coefficient sum vanishes. Choose a prime $p$ dividing none of the finitely many nonzero numbers ${\sum }_{i\in I}{c}_i$. Colour $x\in \mathbb{N}$ by the first nonzero base-$p$ digitIf $x_1,\dots ,x_r$ had one colour, divide the equation by the least power of $p$ occurring among them and reduce modulo $p$. The terms of minimum valuation givefor a nonzero $u$, contradicting the choice of $p$.

For sufficiency, reorder so that $c_1+\cdots +c_t=0$. The standard focusing lemma derived from the Van der Waerden theorem says the following: given a finite colouring, a finite monochromatic solution of the first $j$ blocks of the columns condition can be chosen together with a common difference $d$ so that every bounded translate of every chosen entry by a multiple of $d$ retains its colour. To prove the lemma, refine the colour of $a$ to the finite vector $(\chi (a),\chi (2a),\dots ,\chi (Ra))$, apply van der Waerden to a sufficiently long progression in this refined colouring, and take a common multiple of the finitely many resulting coefficients as $d$.

Start with the zero-sum block $\{1,ldots,t\}$, for which equal variables already solve its contribution. Add each remaining coefficient as a singleton block. In one dimension its block sum is a rational multiple of any fixed nonzero earlier coefficient, so the focusing lemma chooses a bounded translate that cancels this new contribution while preserving the common colour. Induction over the remaining indices gives a monochromatic solution of the full equation. This proves the single-equation form of Rado's theorem.

Now let $a_1,ldots,a_n,b$ be positive and put $A=a_1+\cdots +a_n$. If $b=At$, then $x_1=\cdots =x_n=t$ is a monochromatic solution in every colouring. Conversely, every solution has $x_i\le b/a_i$. Give each integer at most $b/{\min }_i{a}_i$ its own colour and colour all larger integers with one extra colour. A monochromatic solution must have all $x_i=t$, so $b=At$. Thus the inhomogeneous equation is partition regular exactly when $A$ divides $b$.

Solved by gpt-5.6-sol high.