residual r=ax-b ∞-norm

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dulanga
dulanga 2019-3-18
评论： dulanga 2019-3-19
This is my code but i dont get the coorect answer
a = [-5 8 -5; 0 0 12; 3 -7 -3;6 -4 4]
b = [1;1;1;1]
x=a\b
r=a*x-b
x1=sqrt(sum(r.*r))

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dulanga
dulanga 2019-3-19
Would this be correct to get infinty norm
a = [-5 8 -5; 0 0 12; 3 -7 -3;6 -4 4]
b = [1;1;1;1]
x=a\b
r=a*x-b
N = norm(r, Inf)
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John D'Errico
John D'Errico 2019-3-19
That is correct, as long as you are allowed to use the norm function. Things like that are sometimes not allowed in homework. If not, then you need to do as I suggested, as
max(abs( r ))

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John D'Errico
John D'Errico 2019-3-19
Why would you possibly have expected to get the right answer? Was the infinity norm requested? Is it a 2-norm that you computed?
x1=sqrt(sum(r.*r))
https://en.wikipedia.org/wiki/Norm_(mathematics)
If r is the vector of residuals, then what do you want to compute? It seems like you want the maximum of the absolute value of the residuals. How would you do that?

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