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  • Linear Algebra lecture8 note

    Compute solution of AX=b (X=Xp+Xn)

    rank r

    r=m solutions exist

    r=n solutions unique

     


    example:

    image

    若想方程有解,b1,b2,b3需要满足什么条件? 观察矩阵可知,第三行是前两行的和,所以b1+b2=b3

    image

    Solvability Condition on b:

    Ax=b is solvable when b is in C (A)

    If a combination of Rows of A gives zero row, then the same combination of entries of b must give 0

    假设image,则上述矩阵变为:

    image

    To find complete solution to AX=b:

    1.Xp (particular): set all free variables to zero, solve AX=b for pivot variable

    此例中,X2=0,X4=0

    image

    image

    2.Xn(nullspace) 上一节已经解出

    image

    3.X(complete)=Xp+Xn

    image

    以上操作可解释为:

    image

     


    m by n matrix A of rank r(r<=m,r<=n)

    Full column of rank(r=n):

    所有列均有主元; no free variables;  N(A)=zero vector; solution to AX=b is X=Xp which means if solution exists then the solution is unique(0 or 1 solution)

    这种情况实际就是,除zero组合之外,列之间的线性组合无法产生零列

    Full row of rank(r=m):

    所有行均有主元; no zero rows; can solve AX=b for every b; left with n-r(n-m) free variables

    Full rank(r=m=n):

    N(A)=zero vector; R(行最简形)=I(单位矩阵)

     

    summary:

    矩阵的秩决定了方程组解的数目

    image

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  • 原文地址:https://www.cnblogs.com/nanocare/p/6016595.html
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