# Matroid Direct Sum

According to the direct sum theorem of matroid, (1) the union of 2 matroids whose ground sets are disjoint and (2)whose independent set is defined as the union of the independent sets of the two respective matroids, is a matroid.

when we say two matroid in this context, do we refer to the same types of matroids?

• What do you mean by the "type" of a matroid? Aug 14, 2012 at 2:39
• For example In linear algebra we can have a set of vectors as ground sets and the independent set correspond to the linearly independent vectors in the set. In graphs we can define the independent set as the set of edges which does not result to cycles. Aug 14, 2012 at 2:52
• Then the answer is no. The type of the matroids doesn't matter.
– Ido
Aug 14, 2012 at 6:55

Take $A$, a $2\times 2$ matrix as $\begin{bmatrix} 1 & 0\\ 0 & 0\end{bmatrix}$ and Define the Graph $G$ be the complete graph on 3 vertices, meaning $K_3$ with edges called $a$, $b$ and $c$. Then if I call the matroids coming from them with $M_1$ and $M_2$ respectively, we have (index columns of $A$ with $1$ and $2$ from left to right);
$\begin{array}{l} E(M_1)=\{1,2\}\\ I(M_1)=\{\emptyset,\{1\}\}\\ E(M_2)=\{a,b,c\}\\ I(M_2)=\{\emptyset,\{a\},\{b\},\{c\},\{a,b\},\{a,c\},\{b,c\}\} \end{array}$
$\begin{array}{l} E(M):=\{1,2,a,b,c\}\\ I(M)=\{\emptyset,\{1\},\{a\},\{b\},\{c\},\{a,b\},\{a,c\},\{b,c\}\} \end{array}$