Independent Set can be Augmented by Larger Independent Set

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Theorem

Let $M = \struct{S, \mathscr I}$ be a matroid.

Let $X, Y \in \mathscr I$ such that:

$\size X < \size Y$


Then there exists non-empty $Z \subseteq Y \setminus X$ such that:

$X \cup Z \in \mathscr I$
$\size {X \cup Z} = \size Y$


Corollary

Let $B \subseteq S$ be a base of $M$.

Then:

$\exists Z \subseteq B \setminus X : \card{X \cup Z} = \card B : X \cup Z$ is a base of $M$


Proof

Let $\mathscr Z = \set {Z \subseteq Y \setminus X : X \cup Z \in \mathscr I}$

Note that $\O \in \mathscr Z $

So $\mathscr Z \ne \O$


Let $Z_0 \in \mathscr Z : \size {Z_0} = \max \set {\size Z : Z \in \mathscr Z}$


Aiming for a contradiction, suppose:

$\size {X \cup Z_0} < \size Y$

By matroid axiom $(\text I 3)$:

$\exists y \in Y \setminus \paren{X \cup Z_0} : X \cup Z_0 \cup \set y \in \mathscr I$

By choice of $Z_0$ and $y$:

$Z_0 \cup \set y \subseteq Y \setminus X$

So:

$Z_0 \cup \set y \in \mathscr Z$

Then:

\(\displaystyle \size {Z_0 \cup \set y}\) \(=\) \(\displaystyle \size {Z_0} + \size {\set y}\) Corollary to Cardinality of Set Union
\(\displaystyle \) \(=\) \(\displaystyle \size {Z_0} + 1\) Cardinality of Singleton
\(\displaystyle \) \(>\) \(\displaystyle \size {Z_0}\)


This contradicts the choice of $Z_0$.


It follows that:

\(\displaystyle \size Y\) \(\le\) \(\displaystyle \size {X \cup Z_0}\)
\(\displaystyle \) \(=\) \(\displaystyle \size X + \size {Z_0}\) Corollary to Cardinality of Set Union
\(\displaystyle \leadsto \ \ \) \(\displaystyle \size {Z_0}\) \(\ge\) \(\displaystyle \size Y - \size X\)


From Finite Set Contains Subset of Smaller Cardinality:

$\exists Z \subseteq Z_0 : \size Z = \size Y - \size X$

Then:

\(\displaystyle \size {X \cup Z}\) \(=\) \(\displaystyle \size X + \size Z\) Corollary to Cardinality of Set Union
\(\displaystyle \) \(=\) \(\displaystyle \size Y\)


We have:

\(\displaystyle \card Z\) \(=\) \(\displaystyle \card Y - \card X\)
\(\displaystyle \) \(>\) \(\displaystyle 0\) As $\card X < \card Y$
\(\displaystyle \leadsto \ \ \) \(\displaystyle Z\) \(\neq\) \(\displaystyle \O\) Cardinality of Empty Set

$\blacksquare$


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