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:
\(\ds \size {Z_0 \cup \set y}\) | \(=\) | \(\ds \size {Z_0} + \size {\set y}\) | Corollary to Cardinality of Set Union | |||||||||||
\(\ds \) | \(=\) | \(\ds \size {Z_0} + 1\) | Cardinality of Singleton | |||||||||||
\(\ds \) | \(>\) | \(\ds \size {Z_0}\) |
This contradicts the choice of $Z_0$.
It follows that:
\(\ds \size Y\) | \(\le\) | \(\ds \size {X \cup Z_0}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \size X + \size {Z_0}\) | Corollary to Cardinality of Set Union | |||||||||||
\(\ds \leadsto \ \ \) | \(\ds \size {Z_0}\) | \(\ge\) | \(\ds \size Y - \size X\) |
From Finite Set Contains Subset of Smaller Cardinality:
- $\exists Z \subseteq Z_0 : \size Z = \size Y - \size X$
Then:
\(\ds \size {X \cup Z}\) | \(=\) | \(\ds \size X + \size Z\) | Corollary to Cardinality of Set Union | |||||||||||
\(\ds \) | \(=\) | \(\ds \size Y\) |
We have:
\(\ds \card Z\) | \(=\) | \(\ds \card Y - \card X\) | ||||||||||||
\(\ds \) | \(>\) | \(\ds 0\) | As $\card X < \card Y$ | |||||||||||
\(\ds \leadsto \ \ \) | \(\ds Z\) | \(\neq\) | \(\ds \O\) | Cardinality of Empty Set |
$\blacksquare$
Sources
- 1976: Dominic Welsh: Matroid Theory ... (previous) ... (next) Chapter $1.$ $\S 5.$ Properties of independent sets and bases, Theorem $5.1$