目录:
vector space (向量空间)subspace space (子空间)由Ax=b理解column space (列空间)由Ax=0理解null space(零空间),求解Ax=0的主变量及特解矩阵的秩(rank)
1. 向量空间
从字面理解,向量所在的空间,即列向量所处的空间维度。
Definition: The space Rn consists of allcolumn vectorsv with
性质:向量间相加,向量间的数乘以及线性组合仍然在此空间中。
Rn中用R的原因是向量中每个值都是real number,如果向量中每个值都是复数,那么则用
例如:
Here are three vector spaces other than Rn:
M - The vector space of all real 2 by 2 matrices.
Z - The vector space that consists only of a
2.子空间
Definition:A subspace of a vector space isa set ofvectors (including 0) that satisfies two requirements: If
(i)
(ii) cv is in the subspace;
The whole space is a subspace (of itself) including:
(1)The whole space.
(2)Every subspace contains the
(3)Lines through the origin are also subspaces;
(4)The single vector (0,0,0);
例如:
R3的子空间:
(直线L) Any line through
(向量空间R3)The whole space
(平面P) Any plane through
(零向量Z)The single vector
子空间性质:属于(inside)向量空间,且其对数乘、向量相加以及线性组合也是封闭的。
3.列空间
Definition: Thecolumn spaceconsists ofall linear combinations f columns. The combinations are all possible vector Ax. They fill the column space C(A).
The system Ax=b is solvable if and only if b is in the column space of
S=set−of−vectors−in−V
SS=all−combinations−of−vectors−in−S
The subspace SS is the ‘span’ of S, containing all combinations of vectors in
举例:
A=⎡⎣⎢⎢⎢123411112345⎤⎦⎥⎥⎥
A的列空间是
Ax=⎡⎣⎢⎢⎢123411112345⎤⎦⎥⎥⎥⎡⎣⎢x1x2x3⎤⎦⎥=⎡⎣⎢⎢⎢b1b2b3b4⎤⎦⎥⎥⎥
什么情况b使方程有解呢?
》》结论:有解情况,当
深入分析: A的列空间属于
备注:之前在PartI中向量线性组合时(vector combination)提及组合后向量在空间所占位置。
1)对于一个向量
2)对于两个向量
3)对于三个向量
4.零空间
The NULL space of
The null space of A consists of all solutions to
Special solutions
The nummspace consits of all combinations of the special solutions.
整个解的过程: 把A变成
1)Produce zeros above the pivots, by eliminating upward;
2)Produce ones in the pivots, by dividing the whole row by its pivot.
Ax=0,
A=⎡⎣⎢⎢⎢123411112345⎤⎦⎥⎥⎥其零空间为在R3中的线。几个特殊的解其线性组合为一条线,然后其线性组合就构成了方程的解(矩阵A的零空间)。
例子:
解: Ax=0—->Ux=0
A−−>⎡⎣⎢100200222244⎤⎦⎥−−>⎡⎣⎢100200220240⎤⎦⎥=U
5.矩阵的秩
秩:矩阵中主元的个数,为矩阵的秩。(number of pivots)
如上例,A的秩为2.当
x=⎡⎣⎢⎢⎢−2100⎤⎦⎥⎥⎥及⎡⎣⎢⎢⎢−20−21⎤⎦⎥⎥⎥
其也是零空间基(basis)。
解X(A的零空间):特解的线性组合
零空间矩阵,将所有特解作为列的矩阵。
啰嗦下主变量(Pivot variables)
用r表示主变量的个数,
对于
n−r=4−2.自由变量的数量
自由变量,故名思议,可以任意取值,取值后的特解的线性组合即为方程组的解(矩阵的零空间)。一般取得是(0或1)。