Element of Matroid Base and Circuit has a Substitute/Lemma 1

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Theorem

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

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

Let $C \subseteq S$ be a circuit of $M$.

Let $x \in B \cap C$.


Then:

$C \setminus \set x$ is an independent proper subset of $C$


Proof

By definition of circuit:

$C$ is a minimal dependent subset

From Singleton of Element is Subset:

$\set x \subseteq C$ and $\set x \subseteq B$

From matroid axiom $(\text I 2)$:

$\set x \in \mathscr I$

Because $C \notin \mathscr I$:

$\set x \ne C$

Hence:

$\set x \subsetneq C$

From Set Difference with Proper Subset is Proper Subset:

$C \setminus \set x$ is a proper subset of $C$

As $C$ is a minimal dependent subset:

$C \setminus \set x \in \mathscr I$

$\blacksquare$