Free matroid

The free extension of a matroid ${\displaystyle M}$ by some element ${\displaystyle e\not \in M}$, denoted ${\displaystyle M+e}$, is a matroid whose elements are the elements of ${\displaystyle M}$ plus the new element ${\displaystyle e}$, and:

• Its circuits are the circuits of ${\displaystyle M}$ plus the sets ${\displaystyle B\cup \{e\}}$ for all bases ${\displaystyle B}$ of ${\displaystyle M}$.[2]^: 1
• Equivalently, its independent sets are the independent sets of ${\displaystyle M}$ plus the sets ${\displaystyle I\cup \{e\}}$ for all independent sets ${\displaystyle I}$ that are not bases.
• Equivalently, its bases are the bases of ${\displaystyle M}$ plus the sets ${\displaystyle I\cup \{e\}}$ for all independent sets of size ${\displaystyle {\text{rank}}(M)-1}$.