Vector Spaces II: Finite Dimensional Linear Algebra 1


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1 John Nachbar September 2, 2014 Vector Spaces II: Finite Dimensional Linear Algebra 1 1 Definitions and Basic Theorems. For basic properties and notation for R N, see the notes Vector Spaces I. Definition 1. X R N, X, is a vector space iff the following conditions hold. 1. For any x, ˆx X, x + ˆx X. (X is closed under vector addition.) 2. For any x X and any α R, αx X. (X is closed under scalar multiplication.) Since 0 R, if X is a vector space then, by condition (2) of the definition, X contains 0x = (0,..., 0). That is, any vector space must contain the origin. Example 1. R N is itself a vector space. Example 2. The set of points in R 2 given by the graph of y = ax + b, a, b 0 does not contain the origin and is not a vector space. On the other hand, the graph of y = ax is a vector space. Definition 2. Let X R N be a vector space and suppose B X is also a vector space. Then B is a vector subspace of X. Example 3. If X R N is a vector space then it is a vector subspace of R N. Example 4. R 1 is a vector subspace of R 2. But the set [ 1, 1] is not a vector subspace because it is not closed under either vector addition or scalar multiplication (for example, = 2 [ 1, 1]). Geometrically, a vector space in R N looks like a line, plane, or higher dimensional analog thereof, through the origin. A key feature of a vector space X R N is that X can be characterized by listing only a few of its vectors. The characterization is not unique, except in the trivial case X = {0}. These characterizing vectors are said to span the space. Definition 3. Let S = {s 1,..., s r } be a set of r vectors in R N. The span of S is { } span(s) = x R N : λ R r such that x = λ i s i. 1 cbna. This work is licensed under the Creative Commons AttributionNonCommercial ShareAlike 4.0 License. i=1 1
2 Example 5. The span of S = {(1, 0), (0, 1)} is all of R 2, as is easily verified by noting that x = (x 1, x 2 ) = x 1 (1, 0) + x 2 (0, 1). Less obviously S = {(1, 1), (1, 1)} has the same span. I return to this below. The span of S = {(1, 1)} is all points of the form (α, α). Theorem 1. Let S = {s 1,..., s r } be a nonempty subset of R N. Then span(s) is a vector space. Proof. I need to check that the definition of vector space is satisfied. Trivially, span(s) R N. It remains to show that x + ˆx, αx span(s) for any x, ˆx span(s), any α R. Let x, ˆx span(s). Then there are vectors λ, ˆλ R r such that x = r i=1 λ is i and ˆx = r ˆλ i=1 i s i. Then x + ˆx = r i=1 (λ i + ˆλ i )s i span(s). Similarly, for any α R, αx = α r i=1 λ is i = r i=1 (αλ i)s i span(s). The following says that a vector s i S is redundant, in the sense of not increasing span(s) iff s i is itself in the span of the other vectors in S. Theorem 2. Let s i S = {s 1,..., s r } R N. s i span(s \ {s i }) iff span(s) = span(s \ {s i }). Proof. Possibly relabeling the elements of S, suppose that s 1 span(s \ {s 1 }). Since S \ {s 1 } S, span(s \ {s 1 }) span(s). It remains to show that span(s) span(s \ {s 1 }). Since s 1 span(s \ {s 1 }), there is a ρ R r 1 such that s 1 = r i=2 ρ is i. Consider any x span(s). Then there is a λ R r such that x = r i=1 λ is i. Therefore x = λ 1 s 1 + λ i s i i=2 ( ) = λ 1 ρ i s i + λ i s i i=2 i=2 = (λ 1 ρ i + λ i ) s i. i=2 Hence x span(s \ {s 1 }), as claimed. If span(s) = span(s \ {s i }) then, since s i S span(s) = span(s \ {s i }), s i span(s \ {s i }). Given a vector space, one wishes to identify sets S that span that vector space efficiently, that is, S that contain a minimal number of elements. Such sets are independent in the following sense. Definition 4. Let S = {s 1,..., s r } be a subset of R N, S. S is dependent iff there is a λ R r, λ 0, such λ i s i = 0 i=1 2
3 If S is not dependent then it is independent. Note that if S is independent and s i S then s i 0. Theorem 3. Let S = {s 1,..., s r } be a subset of R N. S is independent iff there is no s i S such that s i span(s \ {s i }). Proof. The argument is by contraposition. Possibly relabeling the elements of S, suppose that s 1 span(s\{s 1 }). Then there is a ρ R r 1 such that s 1 = r i=2 ρ is i. Hence 0 = s 1 ρ i s i, which immediately implies that S is not independent. Again, the argument is by contraposition. Suppose S is dependent. Then there is a λ R r such that 0 = r i=1 λ is i but λ 0. Possibly relabeling elements, suppose λ 1 0. Then s 1 = That is, s 1 span({s 2,..., s r }). i=2 i=2 ( λ ) i s i. λ 1 By Theorem 2, an immediate corollary of Theorem 3 is the following. Theorem 4. Let S = {s 1,..., s r } be a subset of R N. S is independent iff there is no s i S such that span(s) = span(s \ {s i }). By Theorem 4, a set S is independent iff it contains no vector that is redundant in the sense that it could be deleted from S without altering the span. This implies that the search for a minimal spanning set should focus on independent sets. In particular, I ask, for a given vector space X, for what independent sets S is span(s) = X? Such sets are called bases for X. Definition 5. Let X be a vector space. S X is a basis for X iff S spans X and S is independent. Remark 1. A basis is also sometimes called a Hamel basis. Suppose S = (s 1,..., s r ) is a basis for X. Then, since S spans X, for any x X there is an λ R r such that x = λ i s i. i=1 λ i is the ith coordinate. For any basis, the coordinate representation is unique (for that basis). If also x = r i=1 ρ is i, so that 0 = x x = r i=1 (λ i ρ i )s i, then the independence of S implies λ i ρ i = 0, or λ i = ρ i for every i. 3
4 When X = R N the standard basis consists of the unit vectors e n = (0,..., 0, 1, 0,..., 0), where the 1 appears in the Nth place. This is indeed a basis for R N. e n spans R N, since, for any x R N, x = N n=1 x ne n. (This is exactly what one means when one writes x = (x 1,..., x N ).) Likewise, the e n are independent, since 0 = N n=1 λ ne n immediately implies (by the definition of e n ), 0 = λ n for all N. In the standard basis, the Nth coordinate is, of course, just x n. It is important to remember that the standard basis for R N is not the only basis for R N. Indeed, except for the trivial case X = {0}, every vector space has an infinite number of bases. Thus, for example, {(2, 1), (1, 2)} forms a basis for R 2. Theorem 8 below provides a tool for checking this assertion. Consider the point (11, 1) (written in the standard basis). In the basis {(2, 1), (1, 2)}, the coordinates get rewritten as (7, 3). This exercise may seem artificial, but finding coordinates in a new, nonstandard, basis is effectively what one does when one solves systems of simultaneous linear equations. Definition 6. Let X be a vector space. dim(x), the dimension of X, is r iff there is an independent set of r vectors in X, but no independent set of r + 1 vectors in X. If dim(x) = r then one expects that any basis for X contains r vectors. I confirm this by first noting that if S spans X and S has t vectors then dim(x) cannot be more than t. (It could be strictly less if S were dependent, in which case some of the t vectors would be redundant.) Let S is the number of elements (vectors) in S. Theorem 5. Let X be a vector space. If S spans X then dim(x) S. Proof. I argue by contraposition. Consider any set of vectors S = (s 1,..., s t ). If dim(x) > S then there is an independent set Z = (z 1,..., z r ) X with Z = r > t = S. I show that S does not span X, and that in particular some z i is not in the span of S. Consider first z 1. If z 1 is not in the span of S, I am done. If z 1 is in the span of S then there is a λ 1 R t such that z 1 = t λ 1 i s i. i=1 Since z 1 0 (since Z is assumed independent), λ 1 i 0 for some i. Possibly relabeling the elements of S, suppose that in fact λ Then s 1 = 1 λ 1 1 z 1 + t i=2 4 ( ) λ1 i λ 1 s i. 1
5 It follows that s 1 span(t 1 ) where T 1 = {z 1, s 2,..., s t }). Therefore span(t 1 ) = span(s). Explicitly, since s 1 span(t 1 ), while s 2,... s t T 1, it follows that span(s) span(t 1 ). Conversely, since z 1 span(s), span(t 1 ) span(s). Next consider z 2. Again, if z 2 span(s) = span(t 1 ), I am done. Otherwise, if z 2 span(t 1 ) then there is a λ 2 R t such that z 2 = λ 2 1z 1 + t λ 2 i s i. Since z 2 0 and since Z is assumed independent (hence, in particular, z 2 λ 2 1 z1 ), λ 2 i 0 for some i > 1. Possibly relabeling the elements of S, suppose that in fact λ As above, I conclude that T 2 = {z 1, z 2, s 3,..., s t } has the same span as T 1 and hence the same span as S. Proceeding in this way for i = 1, 2,..., t, I either uncover a z i not in the span of S or I find that the set T t = {z 1,..., z t } Z has the same span as S. Since r = Z > t, there is a z t+1 Z. Since Z is independent, z t+1 is not in the span of T t (by Theorem 3), hence z t+1 is not in the span of S. Thus S does not span X. i=2 Theorem 6. Let X be a vector space. If S is a basis for X then dim(x) = S. Proof: Suppose S is a basis for X. Since S is independent, dim(x) S. On the other hand, by Theorem 5, since S spans X, dim(x) S. Hence dim(x) = S. The following is then an immediate corollary. Theorem 7. dim(r N ) = N. Theorem 8. Let X be a vector space. Suppose that dim(x) = r. 1. If S X and S = r then span(s) = X iff S is independent. 2. X has a basis, and every basis has r vectors. 3. If S X is independent then there is a Ŝ X such that S Ŝ and Ŝ is a basis for X. Proof. 1. I argue by contraposition. Suppose that S is dependent. Then, by Theorem 4, I can remove an element of S without changing the span. Since dim(x) = r, the contrapositive of Theorem 5 implies that S does not span X. Suppose that S is independent. Consider any x X. Since dim(x) = r, {s 1,..., s r, x} is dependent. Then there is an λ R r+1, λ 0, such that 0 = r i=1 λ is i + λ r+1 x. Since S is independent, λ r+1 0. Therefore x = r i=1 ( λ i/λ r+1 )s i, hence x span(s). 5
6 2. Since dim(x) = r there is an independent set S X with S = r. By (1), S spans X, hence S is a basis for X. That every basis has r vectors follows from Theorem If S = r simply set Ŝ = S. Otherwise, S = t < r. Since S is independent, I know from (1) that S does not span X. Therefore choose any x 1 X \ span(s). Form the set T 1 = {s 1,..., s t, x 1 }. To see that this set is independent, consider any point (λ 1,..., λ t, ρ) R t+1 such that 0 = t λ i s i + ρx 1. i=1 If ρ 0 then x 1 span(s). Since I have assumed x 1 span(s), ρ = 0. Since S is independent, this implies λ i = 0. Hence, T 1 is independent. Continuing in this way, I construct T r t, which is independent, has r vectors, and hence, by (1), is a basis for X. 2 Linear Maps. Definition 7. Let X and Y be vector spaces. L : X Y is a linear map iff both of the following conditions hold. 1. For any x, ˆx X, L(x + ˆx) = L(x) + L(ˆx). 2. For any x X, α R, L(αx) = αl(x). Remark 2. Map is just another word for function. The word map is used most frequently when the target space is something other than R. Choosing α = 0 in part (2) of the definition implies that, for any linear map, L(0) = 0. For example, suppose X = Y = R. Then L(x) = ax is a linear map for any a R. However, B(x) = ax + b, b 0, is not a linear map. Rather, it is called an affine map. Definition 8. Let X and Y be vector spaces. The kernel or null space of a linear map L : X Y, denoted K(L), is the the zero set of L. That is, K(L) = L 1 (0) = {x X : L(x) = 0}. The kernel plays a central role in much of the analysis to follow. Theorem 9. Let X and Y be vector spaces. Let L : X Y be linear. Then L(X) is a vector subspace of Y and K(L) is a vector subspace of X. 6
7 Proof. Consider any y, ŷ L(X). Since y L(X), there is an x X such that L(x) = y. Similarly, there is an ˆx X such that L(ˆx) = ŷ. Then y + ŷ = L(x) + L(ˆx) = L(x + ˆx). Since x + ˆx X, this implies that y + ŷ L(X). Consider any y L(X) and any α R. Since y L(X), there is an x X such that L(x) = y. Then αy = αl(x) = L(αx). Since αx X, this implies that αy L(X). The proof for K(L) is analogous, so I omit it. Theorem 10. Let X and Y be vector spaces. Let L : X Y be linear. Then L is 11 iff K(L) = {0}. Proof. Consider any x X, x 0. Since L(0) = 0 and since L is 11, L(x) 0. Therefore x K(L), which establishes that K(L) = {0}. Consider any x, ˆx X, x ˆx. Then x ˆx {0} = K(L), hence L(x ˆx) 0, hence L(x) L(ˆx), which establishes that L is 11. Theorem 11. Let X and Y be vector spaces. Let L : X Y be linear. Then L is onto iff dim(l(x)) = dim(y ). Proof. Trivial. Let dim(y ) = r. Let V be a basis for L(X). Since dim(l(x)) = dim(y ), V = r. Since V Y and V is independent, Theorem 8 part 1 implies that V is a basis for for Y, hence L(X) = Y. Hence L is onto. 3 Matrices. A linear map L : R N R M has a natural representation in the standard bases of R N and R M. I represent vectors, in the relevant standard basis, as columns. Thus x R N, x = (x 1,..., x N ), is represented x = For a standard basis element e n R N, L(e n ) is some vector a n R M, x 1. x N a n = (a 1n,... a Mn ) = Let x be any element of R N. Then x = ( N n=1 x N ) ne n. Hence L(x) = L n=1 x ne n = N n=1 x nl(e n ) = N n=1 x na n. It is convenient to represent the set {a 1,..., a N } as 7. a 1n. a Mn.
8 an M N matrix Define A = [ a 1... a N] = Ax = a a 1N..... a M1... a MN N n=1 a 1nx n. N n=1 a Mnx n.. Hence L(x) = Ax. I say that L is represented by the matrix A. Any linear map has a matrix representation, and conversely any matrix represents a linear map. Theorem 12. Let L : R N R M be linear. Then L is continuous Proof. Since L is linear, there is a matrix A such that, for any x R N, L(x) = Ax. The claim then follows from the notes on Continuity. Theorem 12 does not generalize to arbitrary metric vector spaces. Linear functions are not, for example, continuous in R with a pointwise convergence metric. Definition 9. Let L : R N R M be a linear map. The transpose of L is L : R M R N, given by L (x) = A x, where the columns of A are the rows of A: A = If A is M n then A is N m. a a M a 1N... a MN. 4 Fundamental Theorem of Linear Algebra Dimension counting arguments play a central role in applications of linear algebra. The canonical example, discussed in Section 5.4, is the analysis of systems of simultaneous linear equations. The central fact used in this application and many others is the following result, sometimes called the Fundamental Theorem of Linear Algebra. Theorem 13 (Fundamental Theorem of Linear Algebra). Let X and Y be vector spaces. If L : X Y is linear then dim(k(l)) + dim(l(x)) = dim(x). 8
9 Proof. Let dim(k(l)) = t and let W = {w 1,..., w t } be a basis for K(L). Let dim(l(x)) = r and let Z = {z 1,..., z r } be a basis for L(X). Since z j L(X), there is a v j X such that L(v j ) = z j. If I can show that S = {w 1,..., w t, v 1,..., v r } is a basis for X then dim(x) = t + r, and I am done. To show that S is independent, suppose 0 = t i=1 λ iw i + r j=1 ρ jv j. I must show that λ i = 0 for all i and ρ j = 0 for all j. Note that 0 = L(0) = L( t i=1 λ iw i + r j=1 ρ jv j ) = t i=1 λ il(w i ) + r j=1 ρ jl(v j ) = r i=1 ρ jz i (since w i K(L) for each i). Since Z is independent (it is a basis), this implies ρ j = 0 for all j. Hence 0 = t i=1 λ iw i. Since W is independent (it is a basis), λ i = 0 for all i. To show that S spans X, let x be any element of X. I must show that x is in the span of S. Since L(x) L(X), and since Z spans L(X), there is a ρ R r such that ( r j=1 ρ jv j ). Let v = r j=1 ρ jv j. Then L(x) = r j=1 ρ jz j = r j=1 ρ jl(v j ) = L x = (x v ) + v. But L(x) = L(v ), so L(x v ) = 0, or x v K(L). Since W spans K(L), there is a λ R t such that x v = t i λ iw i. x = t i λ iw i + r j=1 ρ jv j. Thus x span(s). Thus, Theorem 14. Let X and Y be vector spaces with dim(x) = dim(y ). Let L : X Y be linear. Then L is 11 iff L is onto. Proof. If L is 11 then K(L) = {0}. From Theorem 13, dim(l(x)) = dim(x) = dim(y ), which, by Theorem 11, implies that L is onto. Similarly, if L is onto then dim(l(x)) = dim(y ) = dim(x), which implies K(L) = {0}, hence, by Theorem 10, L is The Structure of Linear Maps. Theorem 13 implies that linear maps must have a very particular structure, formalized in Theorem 15 below. To improve the flow of the narrative, I have divided this section into subsections. Section 5.1 gives the basic definition and results. Section 5.2 reinterprets Theorem 15 in terms of matrices and provides a concrete illustration. Section 5.3 discusses the interpretation of Theorem 15. Section 5.4 applies this machinery to the analysis of systems of simultaneous linear equations. All proofs are collected in Section The basic result. Consider any vector space X and any P, Q X. Define P + Q = {x X : p P, q Q such that x = p + q}. Definition 10. Let P and Q be vector subspaces of a vector space X. P and Q are orthogonal iff p q = 0 for any p P, q Q. P and Q are orthogonal complements iff they are orthogonal and P + Q = X. 9
10 If P and Q are orthogonal complements, I refer to P and Q as a decomposition of X. Similarly, if x = p+q with p P and q Q, I refer to p and q as a decomposition of x. Example 6. For X = R 2, the horizontal and vertical axes are orthogonal complements. So are the spaces P spanned by (1, 1) and Q spanned by (1, 1). The main result of this section is the following theorem. Theorem 15. Let X and Y be vector spaces and let L : X Y be linear. 1. L (Y ) and K(L) are orthogonal complements. Likewise, L(X) and K(L ) are orthogonal complements. 2. dim(l (Y )) = dim(l(x)). Moreover, L maps the set L (Y ) 11 onto the set L(X). 3. For any y L(X), there is an x y L (Y ) such that L 1 (y) = K(L) + {x y }. 5.2 An application to matrices. Let A be any M N matrix and let L : R N R M be the linear map represented by A. L(R N ) is the vector subspace of R M spanned by the columns of A. Accordingly, it is referred to as the column space of A. Similarly, L (R M ) is the space spanned by the rows of A and is, accordingly, referred to as the row space of A. The column rank of A is the number of linearly independent columns of A, which equals dim(l(r N )). The row rank of A is the number of linearly independent rows of A, which equals dim(l (R M )). It follows from (2) of Theorem 15 that for any matrix A, the row rank equals the column rank, which I henceforth refer to simply as the rank of A, written rank(a). By way of illustration, let X = Y = R 2. Suppose [ ] 0 2 A = 0 1 and let L be the corresponding linear map. Note that [ ] A 0 0 =. 2 1 The column space, L(R 2 ), is one dimensional (hence rank(a) = 1) and is spanned by (2, 1). The row space, L (R 2 ), is likewise one dimensional and is spanned by (0, 2). One can verify that the kernel K(L) is one dimensional and is spanned by (1, 0). Finally, one can verify that K(L ) is one dimensional and is spanned by ( 1, 2). The various spaces are illustrated in Figure 1. If the linear map L : R N R N is invertible then let A 1 be the matrix representation of L 1. Linear algebra textbooks provide effective procedures for computing 10
11 L (R 2 ) K(L ) L(R 2 ) 0 K(L) 0 Figure 1: An illustration of Theorem 15. A 1 explicitly. For the moment, I wish only to note that a general function L is invertible if and only if it is 11 and onto. By Theorem 14, if L is a linear function then it suffices to check that L is onto, which means dim(l(r N )) = N. Thus A is invertible if and only if the rank of A is N, a condition for invertibility familiar from elementary linear algebra. 5.3 The interpretation of Theorem 15. Theorem 15 implies that the behavior of a linear map L : X Y is characterized by, first, the decomposition of X into the orthogonal complements K(L) and L (Y ) and, second, by the behavior of L on L (Y ). More specifically, for any x k K(L), consider L (Y ) + {x k }, which is a copy of L (Y ) shifted parallel to L (Y ) by the vector x k. See Figure 2 for an illustration using the example of Section 5.2. For any x L (Y ), L(x + x k ) = L(x). Thus, X can be chopped up into parallel copies of L (Y ), and the behavior of L on each copy is effectively the same as it is on L (Y ). In particular, L maps each such copy 11 onto L(X) (by (2) of Theorem 15). One can also view X as being chopped up into the preimage sets of L, that is, sets of the form L 1 (y) for each y L(X). Of course, L 1 (0) = K(L). Part (3) of Theorem 15 states that every preimage set of L is simply a copy of K(L), translated by some vector in L (Y ). This is illustrated in Figure 3. Translates of vector spaces are called linear manifolds. Thus L 1 (y) is a linear manifold for each y L(X). One can unambiguously define the dimension of L 1 (y) to be the same as the dimension of K(L), namely dim(x) dim(l(x)). A linear manifold in R N of dimension N 1 is called a plane. For N 4, one 11
12 L (R 2 ) L (R 2 )+{x k } K(L ) L(R 2 ) K(L) x k 0 0 L L Figure 2: For each x k K(L), L maps L (R 2 ) + {x k } 11 onto L(R 2 ). L 1 (y) = K(L) + {x y } L'(R 2 ) x y K(L') L L(R 2 ) L 1 (0) = K(L) y O O Figure 3: For each y L(R 2 ), there is an x y such that L 1 (y) = K(L) + {x y }. 12
13 often sees the word hyperplane rather than plane, but word plane is still correct. In applications, planes often arise as the preimage of a linear map. Specifically, suppose that L : R N R is linear and onto. Then for any y R, L 1 (y) has dimension equal to dim(k(l)) = dim(r N ) dim(l(r N )) = N 1, and is thus a plane. Note that L is represented by a 1 N matrix of the form A = [a 1... a N ] The transpose of A is A = a 1. a N, which can be viewed as a vector in R N, call it v. Thus A = v and the canonical plane, namely K(L), is the set of vectors x R N that are orthogonal to v. 5.4 Application: simultaneous equations, kernels, and planes. A set of M linear equations in N unknowns can be written in the form Ax = y, where A is M N and the ith row of A gives the coefficients for the ith equation. Note that x R N while y R M. This sort of problem arises frequently in economic applications. Geometrically, Ax = y if y lies in the column space of A, in which case x is the coordinate representation for y in terms of the columns of A. One is interested in knowing whether any solutions x exist, and if so, how many. Is there any solution to Ax = y? The answer is yes if and only if y is in the column space of A. A sufficient condition for y to be in the column space of A is rank(a) = M, a necessary condition for which is that N M (at least as many unknowns as equations). If M > N (more equations than unknowns) then Ax = y is said to be overdetermined. If M > rank(a) then Ax = y typically has no solutions; the notion of typically can be formalized but I do not do so here. How many solutions are there to Ax = y? The set of solutions is L 1 (y). Assume that this set is not empty. Then L 1 (y) is a linear manifold of dimension equal to the dimension of the kernel. By the Fundamental Theorem of Linear Algebra, this dimension is N dim(l(r N )) = N rank(a). The solution is unique iff this dimension is zero if and only if L is 11 if and only if rank(a) = N, a necessary condition for which is that M N (at least as many equations as unknowns). If N > M (more unknowns than equations) then Ax = y is said to be underdetermined. In summary, if M > rank(a) then typically there is no solution to Ax = y, but if a solution exists then it is unique. If N > rank(a) then there are a continuum of solutions; indeed L 1 (y) is a linear maniforld of positive dimension. If N = M = rank(a) then there is a unique solution. 13
14 The preceding analysis was in terms of the columns of A. One can also analyze Ax = y in terms of the rows of A. Suppose M = 1 and that A has full rank, namely 1. Then for any y R, L 1 (y) is an N 1 dimensional linear manifold, that is, a plane. If M = 2 and A has rank 2 then for any y R 2, L 1 (y) is the intersection of two planes, one corresponding to the first equation (i.e. the first row of A) and the other corresponding to the second equation. From Theorem 13 and Theorem 15, this intersection must be of dimension N 2. Thus, having two equations rather than one drops the dimension of L 1 (y) by one. By way of example, suppose N = 3, so that each plane is twodimensional. Then their intersection is onedimensional (a line). The only thing that might possibly go wrong is if the two planes are parallel, in which case there is no intersection at all unless the planes exactly coincide. But the planes cannot be parallel if the two rows of A are independent, as must be the case if the rank is two. This is intuitive and it is confirmed by Theorem 13 and Theorem 15. And similarly if M = 3. L 1 (y) is now the intersection of 3 planes, and Theorem 13 and Theorem 15 tell us that if the rank of A is three then the dimension of this intersection is N 3. Again, adding another equation drops the dimension of L 1 (y) by 1. And so on for M = 4, 5,... until M = N, at which point Theorem 13 and Theorem 15 imply that if the rank of A is N then L 1 (y) has dimension zero, meaning that the intersection is a singleton. If I try to add one more equation, so that M = N + 1 > N, then it is no longer possible for A to have rank M. In a sense that can be formalized, although I do not do so, the intersection of M > N planes in R N is typically empty. This sort of dimension counting applies more generally. In general, given two linear manifolds in R N of dimensions c and d, if c + d N then a typical intersection (and I won t formalize typical, although a formalization is possible) has dimension c + d N. This follows from the fact that any linear manifold can be described as the preimage of a linear map. In particular, one can easily show that for any linear manifold in R N, if the manifold has dimension c then there is a linear map L that maps from R N onto R N c such that the linear manifold is a preimage of L. (Note that the preimage of such a map has dimension N (N c) = c, as desired.) The manifold can, in turn, be viewed as the intersection of N c planes in R N, generated by the N c rows in the matrix representation of L. The intersection of two manifolds of dimension c and d is thus equivalent to the intersection of (N c) + (N d) rows, which has dimension N (2N c d) = c + d N, provided all rows are independent. For example, the intersection of two planes (each of dimension N 1) has dimension (N 1) + (N 1) N = N 2. If c + d < N then typically the intersection of the manifolds is empty. 5.5 Proofs. I establish Theorem 15 by means of a series of lemmas. The first of these establishes the orthogonal half of part (1) of Theorem
15 Lemma 1. Let X and Y be vector spaces and let L : X Y be linear. Then L (Y ) and K(L) are orthogonal. Similarly, L(X) and K(L ) are orthogonal. Proof. Let A represent L, hence A represents L. Consider any x K(L). Then Ax = 0. This means that x is orthogonal to each row of A, which means that x is orthogonal to each column of A. Since L (Y ) is spanned by the columns of A, Ax = 0 implies that x is orthogonal to every element in L (Y ), as was to be shown. Similarly for L(X) and K(L ). To show that K(L) and L (Y ) are in fact orthogonal complements I need to show that the union of the bases for K(L) and L (Y ) span all of X. As a first step in doing so, I establish the following general fact about orthogonal spaces. Lemma 2. Let P and Q be vector subspaces of a vector space X. If P and Q are orthogonal then the union of any basis for P and any basis for Q is independent. Proof. Let V = {v 1,..., v r } be any basis for P and let W = {w 1,..., w t } be any basis for Q. Suppose that 0 = t λ i v i + ρ j w j. i=1 j=1 Let p = r i=1 λ iv i P and let q = t j=1 ρ jw j Q. Then p + q = 0, hence 0 = (p + q) (p + q) = p p + 2p q + q q = p p + q q (since, by assumption, p and q are orthogonal). Since p p 0, with equality only if p = 0, and similarly for q q, it follows that p = q = 0. But this implies, since V and W are bases, that λ i = 0 for all i and ρ j = 0 for all j, as was to be shown. The following is then immediate. Lemma 3. Let P and Q be orthogonal vector subspaces of a vector space X. Then dim(p ) + dim(q) dim(x). To prove part (1) of Theorem 15, it thus remains to show that dim(k(l)) + dim(l (R M )) = dim(x). As a step toward establishing this, I first establish the first half of part (2) of Theorem 15. Lemma 4. Let X and Y be vector spaces and let L : X Y be linear. dim(l(x)) = dim(l (Y )). Proof. From Theorem 13 Then dim(x) = dim(l(x)) + dim(k(l)) (1) dim(y ) = dim(l (Y )) + dim(k(l )). (2) 15
16 From Theorem 3 dim(x) dim(l (Y )) + dim(k(l)) (3) dim(y ) dim(l(x)) + dim(k(l )). (4) Equation (1) and inequality (3) together imply dim(l(x)) dim(l (Y )). Equation (3) and inequality (4) together imply dim(l (Y )) dim(l(x)). Combining, these imply as was to be shown dim(l(x)) = dim(l (Y )), Proof of part (1) of Theorem 15. Orthogonality was established in Lemma 1. In view of Lemma 2, to show that K(L) and L (Y ) are orthogonal complements, it suffices to show that But by Theorem 13, dim(k(l)) + dim(l (Y )) = dim(x). dim(k(l)) + dim(l(x)) = dim(x) and by Lemma 4, dim(l (Y )) = dim(l(x)), and so the claim follows. The proof that K(L ) and L(X) are orthogonal complements is analogous. Proof of part (2) of Theorem 15. The first half of part (2) was established in Lemma 4. Define ˆL : L (Y ) L(X) by ˆL(x) = L(x) for any x L (Y ). I must show that ˆL is 11 and onto. By Theorem 14, it suffices to show that ˆL is 11. Consider any x, x L (Y ). Suppose that L(x) = L(x ). Then ˆL(x x ) = 0. Since L (Y ) is a vector space, x x L (Y ). Since 0 = ˆL(x x ) = L(x x ), x x K(L). But L (Y ) and K(L) are orthogonal, hence L (Y ) K(L) = {0}. (The latter follows from Lemma 1. More directly, suppose P and Q are orthogonal and let p P Q. Then p p = 0, which implies p = 0.) Therefore, x x = 0, or x = x. Since x and x were arbitrary, this establishes that ˆL is 11 and hence also onto. Proof of part (3) of Theorem 15. Consider any y L(X). Since L maps L (Y ) 11 onto L(X), there is a unique element of L (Y ), call it x y, such that L(x y ) = y. Consider any x L 1 (y). I must show that there is an x k K(L) such that x = x y +x k. Since K(L) and L (Y ) are orthogonal complements, there is an x L (Y ) and a x K(L) such that x = x + x. But y = L(x) = L(x + x) = L(x ) + L( x) = L(x ), since L( x) = 0. It follows that x = x y, and hence x k = x. 16
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