# Thread: linear combination problem dealing with uniqueness

1. ## linear combination problem dealing with uniqueness

Let $V$ be a vector space and $S \subset V$ with the property that whenever $v_1,v_2,...,v_n \in S$ and $a_1{v_1}+a_2{v_2}+...+a_n{v_n} = 0$, then $a_1 = a_2 = ...=a_n = 0$. Prove that every vector in the span of $S$ can be uniquely written as a linear combination of vectors in $S$.

2. Originally Posted by lllll
Let $V$ be a vector space and $S \subset V$ with the property that whenever $v_1,v_2,...,v_n \in S$ and $a_1{v_1}+a_2{v_2}+...+a_n{v_n} = 0$, then $a_1 = a_2 = ...=a_n = 0$. Prove that every vector in the span of $S$ can be uniquely written as a linear combination of vectors in $S$.
So, let $s \in span \, S$ such that $s = a_1{v_1}+a_2{v_2}+...+a_n{v_n}$ and $s = b_1{v_1}+b_2{v_2}+...+b_n{v_n}$. Then $a_1{v_1}+a_2{v_2}+...+a_n{v_n} = b_1{v_1}+b_2{v_2}+...+b_n{v_n}$. This means that $(a_1-b_1){v_1}+(a_2-b_2){v_2}+...+(a_n-b_n){v_n} = 0$. From the hypothesis, $a_i - b_i = 0, \, \forall i=1,2,...,n$. Therefore, $a_i = b_i, \, \forall i=1,2,...,n$