residual r=ax-b ∞-norm

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))

[Walter Roberson]

what is the definition of infinity norm?

Answer 1

[dulanga]

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)

[Torsten]

Apart from the rounding of the residual required for your homework, it's ok.

[dulanga]

thanks

[John D'Errico]

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 ))

[dulanga]

noted

Answer 2

[John D'Errico]

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?