# Math Help - Basis definition

1. ## Basis definition

As you know, the definition of a basis is that it is an independent spanning set,

which means you need two condition for it to be true...

if U is a subspace of R^n, and B={x1,x2,...,xn}

1.B must be linearly independent

2. B is a spanning set of U

But the thing I don't understand is that how could you have a linear independence for B without having B being a spanning set of U.

for me this is kind of confusing...mostly since I can't really visualize these types of algebra concepts like i do with calculus.

2. Originally Posted by akhayoon
But the thing I don't understand is that how could you have a linear independence for B without having B being a spanning set of U.
Take $B = \{ (0,0,1),(0,1,0) \}$ for the vector space $\mathbb{R}^3$. Now $B$ has linear indepedence. But $B$ does not spam the vector space.

3. so this does not span the subspace because there are not enough vectors or whats the reason???

the book I have really doesn't explain this concept well...

4. Originally Posted by akhayoon
so this does not span the subspace because there are not enough vectors or whats the reason???
How about $(1,0,0)$? Can you express that in terms of $(0,1,0)$ and $(0,0,1)$?

5. no...but that means that they're independent....

but why do those two vectors not span R^3...I don't get it

is there a relationship between the number of vectors and the n of R^n that I can use to help me understand this better...b/c I'm still confused

6. Originally Posted by akhayoon
but that means that they're independent
NO. Independent $v_1,...,v_j$ vectors means if $k_1v_1+...+k_nv_n = \bold{0}$ then $k_1=k_2=...=k_n=0$. They are independent because if $k_1(0,1,0)+k_2(0,0,1) = (0,0,0)$ then $(0,k_1,0)+(0,0,k_2) = (0,0,0)$ then $(0,k_1,k_2) = (0,0,0)$ thus $k_1 = k_2 = 0$.

but why do those two vectors not span R^3...I don't get it
I gave you an example. Consider $(1,0,0)$ it is impossible to express $(1,0,0) = k_1(0,1,0)+k_2(0,0,1)$ because not matter what $k_1,k_2$ you will never be able to get a non-zero entry in the first coordinate. So it cannot span R^3.