Even and Odd Functions: Symmetry Rules

✖️ 1. Algebraic tests for even functions $f(x) = f(-x)$ ( $y$ -axis symmetry)

🪞 Algebraic Test for Even Functions

A function is even if $f(-x) = f(x)$ for all $x$ in the domain.
Even functions have y-axis symmetry (mirror image across the vertical axis).
To test: replace every $x$ with $-x$ and simplify.
If you get back the original function, it's even.
Common examples: $f(x) = x^2$ , $f(x) = \cos(x)$ , $f(x) = |x|$ .

Test $f(x) = x^4 - 3x^2$ : compute $f(-x) = (-x)^4 - 3(-x)^2 = x^4 - 3x^2 = f(x)$ , so it's even.

💡 Even = Mirror across y-axis = Same left and right

1. Algebraic tests for even functions $f(x) = f(-x)$ ( $y$ -axis symmetry)

Algebraic Test for Even Functions

A function $f$ is even if and only if $f(-x) = f(x)$ for all $x$ in its domain. This condition means the function's graph is symmetric about the $y$ -axis.

Intuition: Replacing $x$ with $-x$ leaves the output unchanged, so points $(x, y)$ and $(-x, y)$ both lie on the graph.

Core Rules:

Test: Compute $f(-x)$ and simplify completely. If the result equals $f(x)$ , the function is even.
Domain requirement: The domain must be symmetric about zero (if $x$ is in the domain, so is $-x$ ).
Common examples: $f(x) = x^2$ , $f(x) = \cos(x)$ , $f(x) = |x|$ .

Consequence: Even functions satisfy $\int_{-a}^{a} f(x)\,dx = 2\int_{0}^{a} f(x)\,dx$ when the integral exists.

Example: For $f(x) = 3x^4 - 5x^2 + 7$ , compute $f(-x) = 3(-x)^4 - 5(-x)^2 + 7 = 3x^4 - 5x^2 + 7 = f(x)$ , so $f$ is even.

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✖️ 2. Algebraic tests for odd functions $f(-x) = -f(x)$ (origin symmetry)

🔄 Algebraic Test for Odd Functions

A function is odd if $f(-x) = -f(x)$ for all $x$ in the domain.
Odd functions have origin symmetry (rotate 180 degrees around the origin).
To test: replace every $x$ with $-x$ and simplify.
If you get the negative of the original, it's odd.
Common examples: $f(x) = x^3$ , $f(x) = \sin(x)$ , $f(x) = x$ .

Test $f(x) = 2x^5 - x$ : compute $f(-x) = 2(-x)^5 - (-x) = -2x^5 + x = -(2x^5 - x) = -f(x)$ , so it's odd.

💡 Odd = Spin 180° around origin = Opposite on both sides

2. Algebraic tests for odd functions $f(-x) = -f(x)$ (origin symmetry)

Algebraic Test for Odd Functions

A function $f$ is odd if and only if $f(-x) = -f(x)$ for all $x$ in its domain. This condition corresponds to rotational symmetry about the origin by 180 degrees.

Intuition: Replacing $x$ with $-x$ negates the output, so point $(x, y)$ on the graph implies $(-x, -y)$ is also on the graph.

Core Rules:

Test: Compute $f(-x)$ and simplify. If the result equals $-f(x)$ , the function is odd.
Domain requirement: The domain must be symmetric about zero.
Common examples: $f(x) = x^3$ , $f(x) = \sin(x)$ , $f(x) = \frac{1}{x}$ .
Special property: If $f$ is odd and defined at zero, then $f(0) = 0$ (since $f(0) = -f(0)$ implies $f(0) = 0$ ).

Consequence: For odd functions, $\int_{-a}^{a} f(x)\,dx = 0$ when the integral exists.

Example: For $f(x) = 2x^5 - 4x$ , compute $f(-x) = 2(-x)^5 - 4(-x) = -2x^5 + 4x = -(2x^5 - 4x) = -f(x)$ , so $f$ is odd.

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✖️ 3. Determining parity of sums and products of even/odd functions

➕✖️ Parity of Sums and Products

Even + Even = Even, Odd + Odd = Odd, Even + Odd = Neither.
Even × Even = Even, Odd × Odd = Even, Even × Odd = Odd.
These rules work like algebra with signs.
Use them to quickly determine parity without full algebraic tests.

If $g(x) = x^2$ (even) and $h(x) = x^3$ (odd), then $g(x) + h(x) = x^2 + x^3$ is neither, but $g(x) \cdot h(x) = x^5$ is odd.

💡 Think: Even = +1, Odd = -1, then multiply/add those signs

3. Determining parity of sums and products of even/odd functions

Parity Rules for Combinations

The parity of sums and products of even and odd functions follows algebraic rules derived from their defining properties.

Intuition: Symmetry properties combine predictably: adding symmetric objects preserves symmetry type, while multiplication follows sign rules.

Core Rules:

Sum of two even functions is even: $(f + g)(-x) = f(-x) + g(-x) = f(x) + g(x)$ .
Sum of two odd functions is odd: $(f + g)(-x) = f(-x) + g(-x) = -f(x) - g(x) = -(f + g)(x)$ .
Product of two even functions is even: $(fg)(-x) = f(-x)g(-x) = f(x)g(x)$ .
Product of two odd functions is even: $(fg)(-x) = f(-x)g(-x) = [-f(x)][-g(x)] = f(x)g(x)$ .
Product of even and odd is odd: $(fg)(-x) = f(-x)g(-x) = f(x)[-g(x)] = -f(x)g(x)$ .

Consequence: These rules enable quick parity determination for composite functions without explicit computation.

Example: If $f(x) = x^2$ (even) and $g(x) = x^3$ (odd), then $f(x)g(x) = x^5$ is odd.

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✖️ 4. Identifying functions that are neither even nor odd

🚫 Functions That Are Neither

Most functions are neither even nor odd.
If $f(-x) \neq f(x)$ and $f(-x) \neq -f(x)$ , the function has no symmetry.
Test both conditions; if both fail, it's neither.
Example: $f(x) = x^2 + x$ fails both tests.

For $f(x) = x^2 + 3x$ : compute $f(-x) = x^2 - 3x$ , which is not $f(x)$ and not $-f(x) = -x^2 - 3x$ , so neither.

💡 No mirror, no spin = Neither

4. Identifying functions that are neither even nor odd

Functions with No Parity

A function is neither even nor odd if it fails both tests: $f(-x) \neq f(x)$ and $f(-x) \neq -f(x)$ for at least one $x$ in the domain.

Intuition: Most functions lack symmetry about the $y$ -axis or origin, exhibiting asymmetric behavior.

Core Rules:

Test both conditions: Compute $f(-x)$ and compare to both $f(x)$ and $-f(x)$ . If neither equality holds, the function has no parity.
Common examples: $f(x) = x^2 + x$ , $f(x) = e^x$ , $f(x) = 2^x + 1$ .
Decomposition theorem: Any function with symmetric domain can be uniquely written as $f(x) = f_{\text{even}}(x) + f_{\text{odd}}(x)$ where $f_{\text{even}}(x) = \frac{f(x) + f(-x)}{2}$ and $f_{\text{odd}}(x) = \frac{f(x) - f(-x)}{2}$ .

Consequence: Recognizing lack of parity is essential before applying symmetry-based integration or analysis techniques.

Example: For $f(x) = x^2 + 3x$ , compute $f(-x) = x^2 - 3x$ . Since $f(-x) \neq f(x)$ and $f(-x) \neq -f(x)$ , the function is neither even nor odd.

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✖️ 5. Applications: Analyzing harmonic signals and symmetric waves in acoustics or electromagnetics

🎵 Symmetry in Waves and Signals

Even functions (like $\cos$ ) represent symmetric waves with no phase shift.
Odd functions (like $\sin$ ) represent antisymmetric waves passing through the origin.
In Fourier analysis, even signals use only cosine terms, odd signals use only sine terms.
Symmetry simplifies calculations in acoustics, electromagnetics, and signal processing.
Recognizing parity reduces computation by half in many physics problems.

A vibrating string fixed at both ends has displacement $y(x,t) = A\sin(kx)\cos(\omega t)$ , where $\sin(kx)$ is odd in space.

💡 Even = Cosine waves, Odd = Sine waves

5. Applications: Analyzing harmonic signals and symmetric waves in acoustics or electromagnetics

Symmetry in Wave Analysis

Even and odd function properties simplify Fourier analysis of periodic signals in acoustics and electromagnetics. A signal's symmetry determines which frequency components (cosine or sine) appear in its spectrum.

Intuition: Even signals contain only cosine terms (symmetric waves), while odd signals contain only sine terms (antisymmetric waves), reducing computational complexity.

Core Rules:

Even signals: Fourier series contains only cosine terms and a constant: $f(t) = a_0 + \sum a_n \cos(n\omega t)$ .
Odd signals: Fourier series contains only sine terms: $f(t) = \sum b_n \sin(n\omega t)$ .
Half-wave symmetry: If $f(t + T/2) = -f(t)$ , only odd harmonics appear.
Energy distribution: Symmetry determines power distribution across frequency bands.

Consequence: Identifying signal parity before Fourier decomposition halves the number of coefficients to compute, accelerating spectral analysis in signal processing.

Example: A square wave with $f(-t) = -f(t)$ (odd) has Fourier series $f(t) = \frac{4}{\pi}\sum_{n=1,3,5,...} \frac{\sin(n\omega t)}{n}$ , containing only sine terms.

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Even and odd functions (symmetry)

✖️ 1. Algebraic tests for even functions $f(x) = f(-x)$ ( $y$ -axis symmetry)

🪞 Algebraic Test for Even Functions

1. Algebraic tests for even functions $f(x) = f(-x)$ ( $y$ -axis symmetry)

Algebraic Test for Even Functions

No practice tasks yet

✖️ 2. Algebraic tests for odd functions $f(-x) = -f(x)$ (origin symmetry)

🔄 Algebraic Test for Odd Functions

2. Algebraic tests for odd functions $f(-x) = -f(x)$ (origin symmetry)

Algebraic Test for Odd Functions

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✖️ 3. Determining parity of sums and products of even/odd functions

➕✖️ Parity of Sums and Products

3. Determining parity of sums and products of even/odd functions

Parity Rules for Combinations

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✖️ 4. Identifying functions that are neither even nor odd

🚫 Functions That Are Neither

4. Identifying functions that are neither even nor odd

Functions with No Parity

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✖️ 5. Applications: Analyzing harmonic signals and symmetric waves in acoustics or electromagnetics

🎵 Symmetry in Waves and Signals

5. Applications: Analyzing harmonic signals and symmetric waves in acoustics or electromagnetics

Symmetry in Wave Analysis

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✖️ 1. Algebraic tests for even functions f(x)=f(−x)f(x) = f(-x)f(x)=f(−x) (yyy-axis symmetry)

🪞 Algebraic Test for Even Functions

1. Algebraic tests for even functions f(x)=f(−x)f(x) = f(-x)f(x)=f(−x) (yyy-axis symmetry)

Algebraic Test for Even Functions

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✖️ 2. Algebraic tests for odd functions f(−x)=−f(x)f(-x) = -f(x)f(−x)=−f(x) (origin symmetry)

🔄 Algebraic Test for Odd Functions

2. Algebraic tests for odd functions f(−x)=−f(x)f(-x) = -f(x)f(−x)=−f(x) (origin symmetry)

Algebraic Test for Odd Functions

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✖️ 3. Determining parity of sums and products of even/odd functions

➕✖️ Parity of Sums and Products

3. Determining parity of sums and products of even/odd functions

Parity Rules for Combinations

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✖️ 4. Identifying functions that are neither even nor odd

🚫 Functions That Are Neither

4. Identifying functions that are neither even nor odd

Functions with No Parity

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✖️ 5. Applications: Analyzing harmonic signals and symmetric waves in acoustics or electromagnetics

🎵 Symmetry in Waves and Signals

5. Applications: Analyzing harmonic signals and symmetric waves in acoustics or electromagnetics

Symmetry in Wave Analysis

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Finished reading?

✖️ 1. Algebraic tests for even functions $f(x) = f(-x)$ ( $y$ -axis symmetry)

1. Algebraic tests for even functions $f(x) = f(-x)$ ( $y$ -axis symmetry)

✖️ 2. Algebraic tests for odd functions $f(-x) = -f(x)$ (origin symmetry)

2. Algebraic tests for odd functions $f(-x) = -f(x)$ (origin symmetry)