In 1928 the English mathematician Frank Plumpton Ramsey published his paper On a problem of formal logic [13] in which he proved what would become
known as Ramsey’s Theorem. The paper has led to a large area of combinatorics now known as Ramsey Theory. We shall explore some major results in
Ramsey Theory which all, broadly speaking, find some degree of order within
a large disordered set.
The field of Ramsey Theory has only relatively recently come together to
be viewed as one body and many seemingly basic results are still not known,
and there is no prospect of them being known in the near future.
In 1916 Issai Schur proved that in any finite colouring of the natural numbers
there must exist three monochromatic elements, x, y and z such that x + y = z.
This basic result was generalised by Richard Rado in 1933 to give a characterisation of the homogeneous systems to which a monochromatic solution can be
found in any finite colouring of the natural numbers. We examine these results
in Chapter 4.
Between Schur proving this theorem in 1916 and Rado publishing his theorem in 1933, Ramsey and Van der Waerden published theorems now considered
central to Ramsey Theory.
We shall begin by examining Ramsey’s Theorem, initially for graphs, and
then, more generally, for sets. For example Ramsey’s theorem for graphs states
that in any large enough finitely coloured complete graph there must exist some
large monochromatic substructure. Little is known about the actual orders that
these complete graphs must have to ensure that they contain some particular
monochromatic substructure.
Van der Waerden’s Theorem was proved in 1927, a year earlier than Ramsey’s. Van der Waerden proved that in any finite colouring of the natural
numbers there must exist some monochromatic arithmetic progression with k
terms. We shall give an elegant and short proof based on colour focussing.
Finally we shall turn to Hindman’s Theorem, the most recent theorem which
we shall examine, it was proved in 1974. Hindman’s Theorem states that, for
every finite colouring of the natural numbers there exists some infinite subset
S ⊆ N such that all the finite sums of the elements of S are monochromatic.
Although it is not hard to understand this theorem, it requires some powerful
mathematical tools in its proof. In Chapter 5 we shall build up these ideas
before finally proving the result.