# The Friendship Theorem #

## Definitions and Statement #

- A
`Friendship`

graph is one in which any two distinct vertices have exactly one neighbor in common - A
`Politician`

, at least in the context of this problem, is a vertex in a graph which is adjacent to every other vertex. - The friendship theorem (Erdős, Rényi, Sós 1966) states that every finite friendship graph has a politician.

## Proof outline #

The proof revolves around the theory of adjacency matrices, although some steps could equivalently be phrased in terms of counting walks.

- Assume
`G`

is a finite friendship graph. - First we show that any two nonadjacent vertices have the same degree
- Assume for contradiction that
`G`

does not have a politician. - Conclude from the last two points that
`G`

is`d`

-regular for some`d : ℕ`

. - Show that
`G`

has`d ^ 2 - d + 1`

vertices. - By casework, show that if
`d = 0, 1, 2`

, then`G`

has a politician. - If
`3 ≤ d`

, let`p`

be a prime factor of`d - 1`

. - If
`A`

is the adjacency matrix of`G`

with entries in`ℤ/pℤ`

, we show that`A ^ p`

has trace`1`

. - This gives a contradiction, as
`A`

has trace`0`

, and thus`A ^ p`

has trace`0`

.

## References #

- [P. Erdős, A. Rényi, V. Sós,
*On A Problem of Graph Theory*][erdosrenyisos] - [C. Huneke,
*The Friendship Theorem*][huneke2002]

This property of a graph is the hypothesis of the friendship theorem: every pair of nonadjacent vertices has exactly one common friend, a vertex to which both are adjacent.

## Instances For

A politician is a vertex that is adjacent to all other vertices.

## Instances For

One characterization of a friendship graph is that there is exactly one walk of length 2 between distinct vertices. These walks are counted in off-diagonal entries of the square of the adjacency matrix, so for a friendship graph, those entries are all 1.

This calculation amounts to counting the number of length 3 walks between nonadjacent vertices. We use it to show that nonadjacent vertices have equal degrees.

As `v`

and `w`

not being adjacent implies
`degree G v = ((G.adjMatrix R) ^ 3) v w`

and `degree G w = ((G.adjMatrix R) ^ 3) v w`

,
the degrees are equal if `((G.adjMatrix R) ^ 3) v w = ((G.adjMatrix R) ^ 3) w v`

This is true as the adjacency matrix is symmetric.

Let `A`

be the adjacency matrix of a graph `G`

.
If `G`

is a friendship graph, then all of the off-diagonal entries of `A^2`

are 1.
If `G`

is `d`

-regular, then all of the diagonal entries of `A^2`

are `d`

.
Putting these together determines `A^2`

exactly for a `d`

-regular friendship graph.

If `G`

is a friendship graph without a politician (a vertex adjacent to all others), then
it is regular. We have shown that nonadjacent vertices of a friendship graph have the same degree,
and if there isn't a politician, we can show this for adjacent vertices by finding a vertex
neither is adjacent to, and then using transitivity.

Let `A`

be the adjacency matrix of a `d`

-regular friendship graph, and let `v`

be a vector
all of whose components are `1`

. Then `v`

is an eigenvector of `A ^ 2`

, and we can compute
the eigenvalue to be `d * d`

, or as `d + (Fintype.card V - 1)`

, so those quantities must be equal.

This essentially means that the graph has `d ^ 2 - d + 1`

vertices.

The size of a `d`

-regular friendship graph is `1 mod (d-1)`

, and thus `1 mod p`

for a
factor `p ∣ d-1`

.

Modulo a factor of `d-1`

, the square and all higher powers of the adjacency matrix
of a `d`

-regular friendship graph reduce to the matrix whose entries are all 1.

This is the main proof. Assuming that `3 ≤ d`

, we take `p`

to be a prime factor of `d-1`

.
Then the `p`

th power of the adjacency matrix of a `d`

-regular friendship graph must have trace 1
mod `p`

, but we can also show that the trace must be the `p`

th power of the trace of the original
adjacency matrix, which is 0, a contradiction.

If `d ≤ 1`

, a `d`

-regular friendship graph has at most one vertex, which is
trivially a politician.

If `d = 2`

, a `d`

-regular friendship graph has 3 vertices, so it must be complete graph,
and all the vertices are politicians.

**Friendship theorem**: We wish to show that a friendship graph has a politician (a vertex
adjacent to all others). We proceed by contradiction, and assume the graph has no politician.
We have already proven that a friendship graph with no politician is `d`

-regular for some `d`

,
and now we do casework on `d`

.
If the degree is at most 2, we observe by casework that it has a politician anyway.
If the degree is at least 3, the graph cannot exist.