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Complex modulus restricts to real absolute value
Statement
For every real , write for its canonical image in . Then
Facts & Assumptions
Given: A real number and the complex modulus and real absolute value in the cited definitions. Write for its constant-class image in .
The canonical real embedding sends to its constant class in (The complex numbers as , with the real embedding and imaginary unit ).
The imaginary unit is the class of the polynomial in (The complex numbers as , with the real embedding and imaginary unit ).
Every complex number has unique real coordinates (Real and imaginary parts, complex conjugation, and modulus).
The complex modulus is (Real and imaginary parts, complex conjugation, and modulus).
Real absolute value is nonnegative and equals when and when (Absolute value in an ordered field, Basic properties of the absolute value).
If , then (Squares of nonzero elements are positive); if , then by in a field (Multiplication by zero: ).
Every nonnegative real has a unique nonnegative square root (Square roots exist: a unique with ; the positives are ).
Proof
The canonical image of is the class of the constant polynomial by [F1]. Since is the class of by [F2], this is also the class of , namely . Uniqueness of complex coordinates in [F3] identifies that image with .
If then , and if then [F6] gives . Thus in either case, so [F7] guarantees that its nonnegative square root exists.
By [F5], . If , then ; if , then . In either case , including .
By [F4] and step 1.1, the modulus of the canonical image of is , whose radicand is nonnegative by step 1.2.
Step 2.1 and step 1.3 identify the complex modulus of the canonical image of and as nonnegative square roots of the same . By uniqueness in [F7], they are equal.
Depends on
- The complex numbers as $\mathbb R[x]/(x^2+1)$, with the real embedding and imaginary unit $i$
- Real and imaginary parts, complex conjugation, and modulus
- Absolute value in an ordered field
- Basic properties of the absolute value
- Squares of nonzero elements are positive
- Multiplication by zero: $0 \cdot a = 0$
- Square roots exist: a unique $\sqrt{a} \ge 0$ with $(\sqrt{a})^2 = a$; the positives are $\{x^2 : x \neq 0\}$
Used by
- Equispaced interpolation of Runge's function diverges Example
- BDF2 roots on or outside the unit disk map to the closed right half-plane Lemma
- Realification preserves a complex norm and its metric Lemma
- Both constant-step BDF2 test roots vanish in the stiff limit Proposition
- A chosen algebraic basis identifies a finite-dimensional normed space with a coordinate space Theorem
Dependency tree · two levels
19 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.
Sources
- Tomasz Kochanek, Functional analysis, Lecture 1 (complete five-page lecture notes) (standard reference, not scraped)
- M. Winklmeier, Functional Analysis lecture notes (complete 140-page lecture notes) (standard reference, not scraped)