# Leigh.Samphier/Sandbox/Matroid Satisfies Rank Axioms

## Theorem

Let $S$ be a finite set.

Let $\rho : \powerset S \to \Z$ be the rank function of a matroid on $S$.

Then $\rho$ satisfies the rank axioms:

 $(\text R 1)$ $:$ $\ds \map \rho \O = 0$ $(\text R 2)$ $:$ $\ds \forall X \in \powerset S \land y \in S:$ $\ds \map \rho X \le \map \rho {X \cup \set y} \le \map \rho X + 1$ $(\text R 3)$ $:$ $\ds \forall X \in \powerset S \land y, z \in S:$ $\ds \map \rho {X \cup \set y} = \map \rho {X \cup \set z} = \map \rho X \implies \map \rho {X \cup \set y \cup \set z} = \map \rho X$

## Proof

Let $\rho$ be the rank function of a matroid $M = \struct{S, \mathscr I}$ on $S$

#### $\rho$ satisfies $(\text R 1)$

$\rho$ satisfies $(\text R 1)$ follows immediately from Rank of Empty Set is Zero.

$\Box$

#### $\rho$ satisfies $(\text R 2)$

Let:

$X \subseteq S$
$y \in S$
$\map \rho X \le \map \rho {X \cup \set y}$

Let $Y$ be a maximal independent subset of $X \cup \set y$.

$\card Y = \map \rho {X \cup \set y}$
$Y \setminus \set y \in \mathscr I$

We now consider two cases.

##### Case 1: $y \in Y$

We have:

 $\ds \map \rho X$ $\ge$ $\ds \card{Y \setminus \set y}$ Definition of Rank Function $\ds$ $=$ $\ds \card Y - \card{Y \cap \set y}$ Cardinality of Set Difference $\ds$ $=$ $\ds \card Y - \card{\set y}$ Singleton of Element is Subset $\ds$ $=$ $\ds \card Y - 1$ Cardinality of Singleton $\ds \leadsto \ \$ $\ds \map \rho X + 1$ $\ge$ $\ds \card Y$ Add one to both sides

$\Box$

##### Case 2: $y \notin Y$

We have:

 $\ds \map \rho X + 1$ $>$ $\ds \map \rho X$ $\ds$ $\ge$ $\ds \card{Y \setminus \set y}$ Definition of Rank Function $\ds$ $=$ $\ds \card Y - \card{Y \cap \set y}$ Cardinality of Set Difference $\ds$ $=$ $\ds \card Y - \O$ Intersection With Singleton is Disjoint if Not Element $\ds$ $=$ $\ds \card Y - 0$ Cardinality of Empty Set $\ds$ $=$ $\ds \card Y$

$\Box$

In either case:

$\map \rho X + 1 \ge \card Y = \map \rho {X \cup \set y}$

$\Box$

#### $\rho$ satisfies $(\text R 3)$

Let:

$X \subseteq S$
$y, z \in S$
##### Case 1: $y, z \notin X$

Let $A = X \cup \set y \cup \set z$.

$\map \rho A \ge \map \rho X$

We now prove the contrapositive statement:

$\map \rho A > \map \rho X \implies$ either $\map \rho {X \cup \set y} > \map \rho X$ or $\map \rho {X \cup \set z} > \map \rho X$

Let $\map \rho A > \map \rho X$.

Let $Y$ be a maximal independent subset of $X$.

Let $Z$ be a maximal independent subset of $A$.

$\card Y = \map \rho X < \map \rho A = \card Z$

From Independent Set can be Augmented by Larger Independent Set there exists non-empty set $Z' \subseteq Z \setminus Y$:

$Y \cup Z' \in \mathscr I$
$\card{Y \cup Z'} = \card Z$

$y, z \notin Z'$

We have:

 $\ds Z'$ $=$ $\ds Z' \cap Z$ $\ds$ $\subseteq$ $\ds Z' \cap A$ $\ds$ $=$ $\ds Z' \cap \paren{X \cup \set y \cup \set z}$ $\ds$ $=$ $\ds \paren{Z' \cap X} \cup \paren{Z' \cap \paren{\set y \cup \set z} }$ $\ds$ $=$ $\ds \paren{Z' \cap X} \cup \O$ $\ds$ $=$ $\ds \paren{Z' \cap X}$ $\ds$ $\subseteq$ $\ds X$ $\ds \leadsto \ \$ $\ds Y \cup Z'$ $\subseteq$ $\ds X$

Hence:

 $\ds \card{Y \cup Z'}$ $=$ $\ds \card Z$ $\ds$ $=$ $\ds \map \rho A$ $\ds$ $>$ $\ds \map \rho X$ $\ds$ $\ge$ $\ds \card{Y \cup Z'}$

Hence:

either $y \in Z'$ or $z \in Z'$

Without loss of generality, assume $y \in Z'$.

$Y \cup \set y \in \mathscr I$

We have:

 $\ds \map \rho {X \cup \set y}$ $\ge$ $\ds \card{Y \cup \set y}$ Definition of Rank Function (Matroid) $\ds$ $=$ $\ds \card Y + \card{\set y}$ Corollary of Cardinality of Set Union $\ds$ $=$ $\ds \card Y + 1$ Cardinality of Singleton $\ds$ $>$ $\ds \card Y$ $\ds$ $=$ $\ds \map \rho X$ Definition of Rank Function (Matroid)

This proves the contrapositive statement.

$\Box$

##### Case 2: Either $y \in X$ or $z \in X$

Without loss of generality, assume $y \in X$.

Let:

$\map \rho {X \cup \set y} = \map \rho {X \cup \set z} = \map \rho X$.

We have:

$X \cup \set y \cup \set z = X \cup \set z$

Hence:

$\map \rho {X \cup \set y \cup \set z} = \map \rho {X \cup \set z} = \map \rho X$

$\Box$

In either case:

$\map \rho {X \cup \set y} = \map \rho {X \cup \set z} = \map \rho X \implies \map \rho {X \cup \set y \cup \set z} = \map \rho X$

$\blacksquare$