# Why does if A is a spanning tree which doesn't have $e_1$ then $A\bigcup\{e_1\}$ has a unique cycle?

I am studying the algorithm of Sollin and we recently studied a lemma:

Let be G a graph which values are diffferent on the edges.

• We sort the edges $e_1,e_2,...e_m$ such as $v(e_i)<v(e_j)$
• Every tree of minimal value has at least $e_1$ et $e_2$

Proof

let be A a spanning tree which doesn't have $e_1$

$A\bigcup\{e_1\}$ has a unique cycle

let be $e_i \in$ cycle, $e_i\ne e_1$ then $A\bigcup\{e_1\}-\{e_i\}$ is a tree which value is < $v(A)$ because $v(e_i)>v(e_1)$

I don't understand why would $A\bigcup\{e_1\}$ would have a unique cycle? Do you have a justification? A graphical example?

Lets assume that there's no cycle after adding $e1$. Let $e1$ = $(v1, v2)$. This means that before adding $e1$ there wasn't any path that connects $v1$ and $v2$, which contradicts to the connectivity of the spanning tree. On the other hand, if there was a path the connects $e1$ and $e2$ (which didn't include $(e1, e2)$) then adding this very edge will result into a cycle, which again contradicts to the definition of a tree.
In a tree, there is exactly one path between any pair of vertices. When you add edge e1 you close that unique path to form a unique cycle.