Question

(a) Compute the $\mathrm{L}^{\infty}$ norm on $[0,1]$ of the functions $f(x)=\frac{1}{3}-x$ and $g(x)=x-x^2$. (b) Verify the triangle inequality for these two particular functions.

    (a) Compute the $\mathrm{L}^{\infty}$ norm on $[0,1]$ of the functions $f(x)=\frac{1}{3}-x$ and $g(x)=x-x^2$.
(b) Verify the triangle inequality for these two particular functions.
Applied Linear Algebra (Undergraduate Texts in Mathematics)
Applied Linear Algebra (Undergraduate Texts in Mathematics)
Peter J. Olver,… 2nd Edition
Chapter 3, Problem 4 ↓

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- The $\mathrm{L}^{\infty}$ norm, or supremum norm, of a function $f$ on an interval $[a, b]$ is defined as $\|f\|_{\infty} = \sup_{x \in [a, b]} |f(x)|$. - For $f(x) = \frac{1}{3} - x$, we need to find the maximum value of $|f(x)|$ for $x \in [0, 1]$. - Calculate  Show more…

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(a) Compute the $\mathrm{L}^{\infty}$ norm on $[0,1]$ of the functions $f(x)=\frac{1}{3}-x$ and $g(x)=x-x^2$. (b) Verify the triangle inequality for these two particular functions.
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Key Concepts

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L? Norm (Supremum Norm)
The L? norm, also known as the supremum norm, measures the maximum absolute value of a function over a given domain. In the setting of continuous functions on a closed interval, this norm is well-defined because the function is guaranteed to reach its maximum and minimum values by the Extreme Value Theorem. This concept is critical in functional analysis and helps in assessing the 'size' or 'magnitude' of functions uniformly over their entire domain.
Triangle Inequality in Normed Spaces
The triangle inequality is a fundamental property in normed spaces stating that the norm of the sum of two functions (or vectors) is no greater than the sum of their norms. This inequality underpins the notion of distance in these spaces and is essential for establishing that a given norm indeed induces a metric. Its verification in specific cases helps confirm the consistency and validity of the norm structure within the space.
Optimization on Closed Intervals
Optimization techniques in calculus are used to determine the maximum or minimum values of a function on a closed interval. By considering critical points, where the derivative is zero or undefined, along with the endpoints of the interval, one can guarantee that every continuous function over a closed interval has a maximum and minimum value. This method is especially important when calculating norms like the L? norm, where the supremum is taken over the entire interval.

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Problem 4 (2 points each) Let f and g be bounded functions on [a, b]. (a) Prove the triangle inequality for the uniform norm, ||f + g||u ≤ ||f||u + ||g||u (b) Using your result in (a), prove the reverse triangle inequality for the uniform norm, |||f||u - ||g||u| ≤ ||f - g||u (Hint: Your proof will look very similar to the proof of the reverse triangle inequality for the absolute value)

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