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

## 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
$\set x \subseteq C$ and $\set x \subseteq B$
$\set x \in \mathscr I$

Because $C \notin \mathscr I$:

$\set x \ne C$

Hence:

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

As $C$ is a minimal dependent subset:

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

$\blacksquare$