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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 , with the real embedding and imaginary unit
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:
Square roots exist: a unique with ; the positives are