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# Periodic functions

A periodic function repeats its values at regular intervals, known as the period, which is the distance between consecutive peaks or troughs. Common examples include the sine and cosine functions, fundamental to trigonometry. Understanding periodic functions is crucial for fields such as physics, engineering, and signal processing, where waveforms and cycles are prevalent.

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## Definition of Periodic Functions

Periodic functions are fundamental in mathematics and appear frequently in various fields like physics, engineering, and signal processing.

### What is a Periodic Function?

A periodic function is a function that repeats its values in regular intervals or periods. In simple terms, if you know the function's value at some point, you can predict its value at another point, separated by a fixed length. For example, the sine and cosine functions are classic examples of periodic functions.

Consider the sine function sin(x). It repeats itself every $$2\pi$$ units. This means that for any angle $$x$$, sin(x) = sin(x + 2\pi). Here, the period is $$2\pi$$.

### Mathematical Definition of Periodic Functions

Mathematically, a function f(x) is said to be periodic if there exists a positive number T such that for all x in the domain of f, the equality $$f(x) = f(x + T)$$ holds. The smallest such positive number T is called the fundamental period.

The equation $$f(x) = f(x + T)$$ defines a function that repeats over intervals. Here, f is the function, and T is its period.

Periodic functions are essential in Fourier analysis. Here, complex functions are represented as a sum of simple sine and cosine functions, which are periodic. This representation is widely used in signal processing.

In real-life applications, examples of periodic functions include the motion of a pendulum, the rotation of Earth, and sound waves.

## Examples of Periodic Functions

Periodic functions are ubiquitous in various fields of science and mathematics. They allow you to predict behaviour over time and understand the natural rhythms and repetitive patterns of different phenomena.

### Sine and Cosine Functions

Sine and cosine functions are the most well-known examples of periodic functions. Both functions have a period of $$2\pi$$, which means they repeat their values every $$2\pi$$ radians.

The sine function is defined as $$\sin(x)$$. Its properties include:

• Periodicity: $$\sin(x) = \sin(x + 2\pi)$$
• Amplitude: The maximum value of $$\sin(x)$$ is 1, and the minimum is -1.
• Zeros: $$\sin(x) = 0$$ at x = 0, $$\pi$$, $$2\pi$$, and so on.

Similarly, the cosine function, defined as $$\cos(x)$$, exhibits the following properties:

• Periodicity: $$\cos(x) = \cos(x + 2\pi)$$
• Amplitude: The maximum value of $$\cos(x)$$ is 1, and the minimum is -1.
• Zeros: $$\cos(x) = 0$$ at x = $$\pi/2$$, $$3\pi/2$$, and so on.

A key application of sine and cosine functions is in solving differential equations, especially those that model oscillatory systems. For instance, the equation $$\frac{d^2y}{dt^2} + \omega^2 y = 0$$ describes simple harmonic motion, where the solution can be expressed as a combination of sine and cosine functions: $$y(t) = A \sin(\omega t) + B \cos(\omega t)$$. Here, $$\omega$$ is the angular frequency and determines the period of the motion.

Sine and cosine functions are also used in Fourier series to represent any periodic function as a sum of sine and cosine terms.

### Real-World Examples of Periodic Functions

Periodic functions are not just theoretical concepts; they have practical applications in the real world. Their repetitive nature makes them ideal for describing cyclic phenomena.

Consider the motion of a pendulum. The displacement of the pendulum as a function of time can be modelled using a periodic function. The period of the function corresponds to the time it takes for the pendulum to complete one full swing.

Another classic example is the rotation of the Earth. The Earth's rotation can be modelled as a periodic function with a period of 24 hours. This periodicity results in the regular sequence of day and night.

Periodic functions are also crucial in the field of signal processing. Sound waves, electromagnetic waves, and even economic cycles can be represented using functions that repeat over time. For instance, the sound wave produced by a tuning fork is a simple periodic function. If you plot the displacement of air particles against time, you'll get a sine wave. The frequency of this wave determines the pitch of the sound. In this context, the period is the inverse of frequency: $$T = \frac{1}{f}$$, where $$T$$ is the period and $$f$$ is the frequency.

Periodic functions can also be found in biology. For example, the circadian rhythms that govern sleep-wake cycles in humans are periodic functions with a period of about 24 hours.

## How to Find the Period of a Function

Finding the period of a function is crucial in understanding its behaviour. The period tells you how often the function repeats itself.

### Steps to Determine the Period of a Function

To find the period of a function, you can follow these steps:

• Identify the function: Determine if the function is a trigonometric function such as sine, cosine, or tangent or another type of periodic function.
• Set up the equation: Use the equation $$f(x) = f(x + T)$$, where $$T$$ is the period you need to find.
• Solve for $$T$$: Rearrange the equation to isolate $$T$$ and solve. For trigonometric functions, you often use known periods (for example, $$2\pi$$ for sine and cosine).
• Verify the period: Check that $$f(x) = f(x + T)$$ holds true for multiple values of $$x$$.

Consider the function $$\sin(x)$$. To find its period:

1. Identify that it is a sine function.
2. Set up the equation: $$\sin(x) = \sin(x + T)$$.
3. Since we know the sine function repeats every $$2\pi$$, we get $$T = 2\pi$$.
4. Verify: $$\sin(x) = \sin(x + 2\pi)$$ is true for all $$x$$.
Therefore, the period of $$\sin(x)$$ is $$2\pi$$.

Verifying the period on multiple values of $$x$$ ensures accuracy.

### Formula for Finding the Period

Sometimes, knowing the basic formulas for specific functions helps. The period of the function depends on its type.

The general form of a sine or cosine function is $$a \sin(bx + c) + d$$ or $$a \cos(bx + c) + d$$, where the period $$T$$ is given by $$\frac{2\pi}{|b|}$$.

For the function $$\cos(3x)$$, the period would be calculated as:$|b| = 3$$T = \frac{2\pi}{|b|} = \frac{2\pi}{3}$Therefore, the period of $$\cos(3x)$$ is $$\frac{2\pi}{3}$$.

In some cases, periodic functions can be more complex, requiring in-depth analysis. For example, finding the period of a function like $$\tan\left(\frac{x}{2}\right)$$ involves understanding its fundamental period, which is $$\pi$$. You would calculate it as follows:$f(x) = \tan\left(\frac{x}{2}\right)$Set up the equation: $\tan\left(\frac{x}{2}\right) = \tan\left(\frac{x}{2} + T\right)$Using the known period of the tangent function $$\pi$$:$\frac{x}{2} + T = \frac{x}{2} + n\pi$$T = 2n\pi$For the smallest non-zero period $$n = 1$$,$T = 2\pi$Thus, $\tan\left(\frac{x}{2}\right)$ has a period of $2\pi$.

Use the smallest positive period for practical applications and better understanding.

## Applications of Periodic Functions

Periodic functions have a wide range of applications in various fields, making them an essential tool for understanding and modelling natural and engineered phenomena.

### Periodic Functions in Physics

In physics, periodic functions are crucial for describing oscillatory and wave-like phenomena. Here are some key applications:

Consider the motion of a simple harmonic oscillator, such as a mass on a spring. The displacement as a function of time can be described by a sine or cosine function: $x(t) = A \cos(\omega t + \phi)$

• $$A$$ is the amplitude.
• $$\omega$$ is the angular frequency.
• $$\phi$$ is the phase angle.

The period of such an oscillator is given by $$T = \frac{2\pi}{\omega}$$.

Another important application is in the study of electromagnetic waves. The electric and magnetic fields in a wave can be represented as sinusoidal functions, which are periodic: $E(x, t) = E_0 \sin(kx - \omega t)$ Here, $$E_0$$ is the amplitude, $$k$$ is the wave number, and $$\omega$$ is the angular frequency. These waves have applications in telecommunications, medical imaging, and more.

### Periodic Functions in Engineering

In engineering, periodic functions are used to design and analyse systems that exhibit repetitive behaviour. Some of the common applications include:

In electrical engineering, alternating current (AC) is a type of electrical current that varies periodically. The voltage in an AC circuit can be modelled using a sine function: $V(t) = V_0 \cos(\omega t)$ Here, $$V_0$$ is the peak voltage and $$\omega$$ is the angular frequency of the AC signal.

AC power supply systems work on the principle of periodic functions, typically at a frequency of 50 or 60 Hz, depending on the country.

In mechanical engineering, periodic functions are used to describe the motion of rotating machinery. For example, the angular position $$\theta$$ of a rotating disk can be given by: $\theta(t) = \theta_0 + \omega t$ Here, $$\theta_0$$ is the initial angular position, and $$\omega$$ is the constant angular velocity. Monitoring and controlling these periodic motions are vital for the efficient operation of engines and turbines.

### Use in Other Fields

Periodic functions are not limited to physics and engineering; they also find applications in other fields such as biology, economics, and music.

In biology, many physiological processes are periodic. One such example is the circadian rhythm, which follows a roughly 24-hour cycle. Hormone levels, body temperature, and sleep patterns are governed by these periodic functions.

Understanding biological periodicity can help in the treatment of sleep disorders and the optimisation of medication schedules.

In economics, periodic functions are used to model cyclic behaviours such as business cycles. The Gross Domestic Product (GDP) of a country often shows periodic trends. Analysts use periodic functions to forecast economic conditions and plan fiscal policies accordingly. For example, a simplified economic cycle can be represented as: $GDP(t) = a \cos(\omega t + \phi) + b$ Here, $$a$$ is the amplitude, $$\omega$$ is the angular frequency, $$\phi$$ is the phase, and $$b$$ is an offset representing long-term growth.

In music, periodic functions are used to analyse sound waves. Musical notes can be represented by oscillations with specific frequencies. The waveform of a sound from a musical instrument can often be approximated by a sum of sine and cosine functions, known as a Fourier series.

## Periodic functions - Key takeaways

• Definition of periodic functions: A periodic function repeats its values in regular intervals or periods. Mathematically, it satisfies f(x) = f(x + T) where T is the period.
• Examples of periodic functions: The sine and cosine functions are well-known examples, each having a period of .
• How to find the period of a function: Use the equation f(x) = f(x + T). Solve for T, sometimes using specific formulas like T = 2π/|b| for trigonometric functions.
• Applications of periodic functions: These functions are crucial in fields like physics (e.g., wave motion), engineering (e.g., AC circuits), and biology (e.g., circadian rhythms).
• Properties of periodic functions: Periodic functions have notable characteristics such as repeatability, specific amplitude, and phase shifts; they can be represented as sums of sines and cosines in Fourier analysis.

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What are the properties of periodic functions?
Periodic functions repeat their values at regular intervals, known as the period. Key properties include: the domain is all real numbers, the range is bounded, and they can be represented by Fourier series. Symmetry properties, such as even or odd, can further simplify their analysis.
What is the fundamental period of a periodic function?
The fundamental period of a periodic function is the smallest positive interval \$$T \$$ such that \$$f(x + T) = f(x) \$$ for all values of \$$x \$$ in the domain of the function. It is the length of one complete cycle of the function.
How are periodic functions used in real-world applications?
Periodic functions are used to model phenomena that repeat at regular intervals, such as sound waves, alternating current in electrical engineering, tidal movements in oceanography, and seasonal changes in weather patterns. They help in analysing and predicting cyclic behaviours effectively.
How can you determine whether a function is periodic?
A function is periodic if there exists a positive number \$$T\$$ such that \$$f(x + T) = f(x)\$$ for all \$$x\$$ in its domain. To determine this, identify the smallest such \$$T\$$ (the period) by examining the function's behaviour and finding a consistent repeating pattern.
Can a periodic function have more than one period?
Yes, a periodic function can have more than one period. If a function f(x) has a period T, any multiple of T is also a period. The smallest positive period is called the fundamental period.

## Test your knowledge with multiple choice flashcards

Which differential equation describes simple harmonic motion that can be solved using sine and cosine functions?

What is the period of the function $$\tan\left(\frac{x}{2}\right)$$?

How can the voltage in an AC circuit be modelled?

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