# Relationship between row and column space of a matrix

### Row Space, Column Space, and Null Space

In general, row spaces and column spaces can be very different. For non-square matrices, they will be in different dimension ambient spaces. corresponding to the columns of A. The column space of A is the subspace of \$F^ n Since row operations preserve row space, row equivalent matrices have the . a possible dependence relation the results on rank can be used to give an. Relation to rank. If A is an m × n matrix, to determine bases for the row space and column space of A, we reduce A to a row-echelon form E. 1.

This is actually a review of what we've done the past. We just have to make sure it's closed under addition.

### Column space of a matrix (video) | Khan Academy

So let's say a is a member of our column space. Let's say b is also a member of our column space, or our span of all these column vectors. Then b could be written as b1 times v1, plus b2 times v2, all the way to Bn times Vn. And my question is, is a plus b a member of our span, of our column space, the span of these vectors? Well sure, what's a plus b? I'm just literally adding this term to that term, to get that term. This term to this term to get this term.

And then it goes all the way to Bn and plus Cn times Vn.

Which is clearly just another linear combination of these guys. So this guy is definitely within the span. It doesn't have to be unique to a matrix.

A matrix is just really just a way of writing a set of column vectors. So this applies to any span. So this is clearly a valid subspace. So the column space of a is clearly a valid subspace. Let's think about other ways we can interpret this notion of a column space.

Let's think about it in terms of the expression-- let me get a good color-- if I were to multiply my-- let's think about this. Let's think about the set of all the values of if I take my m by n matrix a and I multiply it by any vector x, where x is a member of-- remember x has to be a member of Rn.

• Row Space and Column Space of a Matrix
• Column space of a matrix
• Row Space, Column Space, and Null Space

It has to have n components in order for this multiplication to be well defined. So x has to be a member of Rn. Let's think about what this means. This says, look, I can take any member, any n component vector and multiply it by a, and I care about all of the possible products that this could equal, all the possible values of Ax, when I can pick and choose any possible x from Rn.

Let's think about what that means. If I write a like that, and if I write x like this-- let me write it a little bit better, let me write x like this-- x1, x2, all the way to Xn. Well Ax could be rewritten as x and we've seen this before-- Ax is equal to x1 times v1 plus x2 times v2, all the way to plus Xn times Vn.

We've seen this multiple times. This comes out of our definition of matrix vector products. Now if Ax is equal to this, and I'm essentially saying that I can pick any vector x in Rn, I'm saying that I can pick all possible values of the entries here, all possible real values and all possible combinations of them.

So what is this equal to? What is the set of all possible? So I could rewrite this statement here as the set of all possible x1 v1 plus x2 v2 all the way to Xn Vn, where x1, x2, all the way to Xn, are a member of the real numbers.

That's all I'm saying here. This statement is the equivalent of this. When I say that the vector x can be any member of Rn, I'm saying that its components can be any members of the real numbers. So if I just take the set of all of the, essentially, the combinations of these column vectors where their real numbers, where their coefficients, are members of the real numbers.

What am I doing? This is all the possible linear combinations of the column vectors of a. So this is equal to the span v1 v2, all the way to Vn, which is the exact same thing as the column space of A.

So the column space of A, you could say what are all of the possible vectors, or the set of all vectors I can create by taking linear combinations of these guys, or the span of these guys. Or you can view it as, what are all of the possible values that Ax can take on if x is a member of Rn?

So let's think about it this way. Let's say that I were to tell you that I need to solve the equation Ax is equal to-- well the convention is to write a b there-- but let me put a special b there, let me put b1. Let's say that I need to solve this equation Ax is equal to b1. And then I were to tell you-- let's say that I were to figure out the column space of A-- and I say b1 is not a member of the column space of A So what does that tell me?

The rows of the product are linear combinations of the rows, Let M and N be matrices over a field F which are compatible for multiplication. The preceding discussion shows that the rows of are linear combinations of the rows of N. Therefore, the rows of are all contained in the row space of N. The row space of N is a subspace, so it's closed under taking linear combinations of vectors. Hence, any linear combination of the rows of is in the row space of N. Therefore, the row space of is contained in the row space of N.

## Row and column spaces

From this, it follows that the dimension of the row space of is less than or equal to the dimension of the row space of N that is. I already have one algorithm for testing whether a set of vectors in is independent.

That algorithm involves constructing a matrix with the vectors as the columns, then row reducing. The algorithm will also produce a linear combination of the vectors which adds up to the zero vector if the set is dependent. If all you care about is whether or not a set of vectors in is independent i. In this approach, you construct a matrix with the given vectors as the rows.

Let V be a finite-dimensional vector space, and let be vectors in V. The object is to determine whether the set is independent. Let M be the matrix whose i-th row is.