Contents

In this article, we dive deep into the heart of the **orthogonal complement**, exploring its definition, properties, and applications. Whether you’re a mathematician seeking to strengthen your understanding or a **curious reader** drawn towards the enchanting world of **linear algebra**, this comprehensive guide on the **orthogonal complement** will illuminate the torchlight.

**Definition of **Orthogonal Complement

In **linear algebra**, the **orthogonal complement** of a subspace **W** of a vector space **V** equipped with an inner product, such as the Euclidean space **$R^n$**, is the set of all vectors in **V** that are orthogonal to every vector in **W**.

If you take any vector **v** from the **orthogonal complement** and any vector **w** from **W**, their inner product will be zero. In other words, **v** and **w** are orthogonal.

In **mathematical notation**, if **W** is a subspace of **V**, the orthogonal complement of **W** is usually denoted as **W⊥ (W perp)**. It is defined as:

W⊥ = { v ∈ V : <v, w> = 0 for all w ∈ W }

Here,** “< , >”** denotes the inner **product** of two vectors.

So, the **orthogonal complement** of **W** consists of all vectors in **V** that are **perpendicular** (or **orthogonal**) to the vectors in **W**. This is a very useful concept in many areas of mathematics and its applications, such as in **computer graphics**, **machine learning**, and **physics**.

Figure-1.

**Properties**

**Orthogonal complements** in vector spaces possess several important properties, making them a critical tool in **linear algebra** and its applications. These properties directly result from the definitions and axioms of vector spaces and inner products. Here are a few of them:

**Non-Negativity**

The **orthogonal complement** of a subspace is always a **subspace** of the original vector space. This means it is **closed under addition and scalar multiplication** and contains the **zero vector**.

**Orthogonality**

Every vector in the **orthogonal complement** is **orthogonal** (perpendicular) to every vector in the original subspace. In other words, the **inner product** of any vector in the orthogonal complement with any vector in the original subspace is zero.

**Self-Orthogonal Complement**

The **orthogonal complement** of a subspace is the **original subspace itself** (given the subspace and its complement are both finite-dimensional). This property is represented in mathematical notation as **(W⊥)⊥ = W**.

**Zero Intersection**

The **intersection** of a **subspace** and its **orthogonal complement** is the zero vector, i.e., **W ∩ W⊥ = {0}**.

**Whole Space**

A **subspace** and its **orthogonal complement** span the whole vector space in a **finite-dimensional vector space**. This is mathematically represented as **V = W ⊕ W⊥**, where ⊕ denotes the **direct sum**.

**Dimensionality**

For **finite-dimensional spaces**, the **dimension** of a subspace plus its orthogonal complement is equal to the dimension of the entire vector space. This is known as the **dimension theorem** for vector spaces.

**Orthogonal to Itself**

If a **subspace** is orthogonal to itself, i.e., **W = W⊥**, it must be the **zero subspace** or the **entire vector space**.

These properties provide a robust framework for manipulating and understanding **orthogonal complements** in various contexts, including **solving systems of linear equations**, **orthogonal projections**, the **Gram-Schmidt process**, and more.

## Evaluating the Orthogonal Complement

The process of finding orthogonal components relies heavily on the concept of **projection**. The **projection** of a vector onto another vector provides the component of the first vector that lies along the second.

The **orthogonal complement** to that **projection** is the component of the first vector orthogonal to the second. Here’s a step-by-step process to find the orthogonal component of a vector **v** concerning another vector **u**:

**Step 1**

Start with vectors** u** and

**v**

in the same **vector space**.

**Step 2**

Calculate the **projection** of ** v** onto

**. The projection of**

`u`

**onto**

`v`

`u`

is given by the formula:`proj_u(v) = ((v·u) / ||u||²) * u`

where** v·u** denotes the

**dot product**of

**and**

`v`

**, and**

`u`

**denotes the**

`||u||`

**norm (length)**of

**. This results in a new vector that lies along**

`u`

**.**

`u`

**Step 3**

**Subtract** the **projection** from ** v**. The difference

**is the**

`v - proj_u(v)`

**orthogonal component**with respect to

`v`

**. This vector is**

`u`

**orthogonal**to

**.**

`u`

Let’s look at an example in** R²**:

Suppose ** v = (4, 3)** and

**. We want to find the**

`u = (2, 2)`

**orthogonal component**of

**with respect to**

`v`

**.**

`u`

**Step 1**

We have our vectors ** v = (4, 3)** and

**.**

`u = (2, 2)`

**Step 2**

Calculate the **projection** of ** v** onto

**:**

`u`

`proj_u(v) = ((v·u) / ||u||²) * u `

`proj_u(v) = ((4*2 + 3*2) / (2² + 2²)) * (2, 2) `

`proj_u(v) = (14/8) * (2, 2) `

`proj_u(v) = (3.5, 3.5)`

**Step 3**

Subtract the **projection** from ** v** to get the

**orthogonal component**:

`v - proj_u(v) = (4, 3) - (3.5, 3.5) `

`v - proj_u(v) = (0.5, -0.5)`

So, the **orthogonal component** of ** v = (4, 3)** with respect to

**is**

`u = (2, 2)`

**. As you can check, the**

`(0.5, -0.5)`

**dot product**of

**and**

`(0.5, -0.5)`

**is zero, confirming that these vectors are indeed**

`(2, 2)`

**orthogonal**.

**Exercise**

**Example 1**

In **R³**, consider the subspace** W** spanned by the vector **(1, 1, 0)**. The **orthogonal complement W⊥** will be spanned by **vectors orthogonal** to **(1, 1, 0)**, such as** (-1, 1, 0)** and **(0, 0, 1)**.

Figure-2.

**Example 2**

In **R²**, consider the subspace** W** spanned by the vector** (3, 4)**. The **orthogonal complement W⊥** will be spanned by any **vector orthogonal** to **(3, 4)**, such as **(-4, 3)**.

**Example 3**

In **R³**, consider the subspace **W** spanned by the vectors **(1, 0, 0)** and **(0, 1, 0)**. The **orthogonal complement W⊥** will be spanned by the vector (0, 0, **1),** which is **orthogonal** to both **(1, 0, 0)** and **(0, 1, 0)**.

Figure-3.

**Example 4**

In **R²**, consider the subspace **W** consisting of the zero vector only (i.e., {0}). The **orthogonal complement W⊥** is the entire space **R²** since every vector in **R²** is** orthogonal** to the zero vector.

**Example 5**

In **R²**, consider the subspace **W = R²**. The **orthogonal complement W⊥** is the zero subspace {0} since only the zero vector is **orthogonal** to every vector in **R²**.

**Example 6**

In **R³**, consider the subspace **W** spanned by the vectors** (1, 2, 3)** and** (4, 5, 6)**.

A vector **v = (x, y, z)** is in **W⊥** if it’s orthogonal to both **(1, 2, 3)** and **(4, 5, 6)**, i.e., if **x + 2y + 3z = 0** and **4x + 5y + 6z = 0**. Solving these gives the orthogonal complement **W⊥** as the set of all **scalar multiples** of **(-1, 2, -1)**.

**Example 7**

In **R⁴**, consider the subspace **W** spanned by the vectors **(1, 0, 0, 0), (0, 1, 0, 0)**, and **(0, 0, 1, 0)**. The **orthogonal complement W⊥** will be spanned by the vector **(0, 0, 0, 1)**, which is **orthogonal** to all vectors in **W.**

**Example 8**

In **R³**, consider the subspace **W** spanned by the vectors **(1, 2, 0)** and **(2, -1, 0)**.

A vector **v = (x, y, z)** is in **W⊥** if it’s **orthogonal** to both **(1, 2, 0)** and **(2, -1, 0)**, i.e., if **x + 2y = 0** and 2**x – y = 0**. Solving these gives the orthogonal complement **W⊥** as the set of all scalar multiples of **(0, 0, 1)**.

**Applications**

The concept of the **orthogonal complement** has widespread applications across numerous fields where **mathematics**, particularly **linear algebra**, comes into play. Here are some of the areas where it proves useful:

**Machine Learning and Data Science**

In **machine learning**, orthogonal complements are used in techniques like **Principal Component Analysis (PCA)** for **dimensionality reduction**. PCA identifies the axes in the feature space along which the data varies the most. These axes are **orthogonal (perpendicular)** to each other, forming an **orthogonal complement** to the remaining, less significant directions in the data.

**Physics**

In **physics**, especially in **quantum mechanics**, the concept of orthogonal complements is used in **Hilbert spaces** to describe **orthogonal states** mutually exclusive. This is important for understanding the behavior and properties of **quantum systems**.

**Computer Graphics and Vision**

**Orthogonal complements** are used in techniques for computing **projections** of points onto a line or a plane. This is extremely important in **3D computer graphics** and **computer vision**, where objects are often transformed and projected onto different planes for **viewing** and **rendering**.

**Signal Processing**

In **digital signal processing**, the concept of **orthogonality** is central to the operation of **separating signals** from **noise** or separating multiple signals from each other. Here, **orthogonal complements** can often be used to** isolate** the **component** of a signal that’s **orthogonal** to the noise.

**Engineering**

In **control theory** (a part of engineering), a **system’s controllable** and **uncontrollable subspaces** are **orthogonal complements** of each other. Understanding these can help design control systems like **cars**, **planes**, or **electronics**.

**Economics and Finance**

In **portfolio theory**, the risk of a **portfolio** can be **decomposed** into **systematic** and **unsystematic risks**, which are **orthogonal** to each other. Similarly, in **econometrics**, the **error term** in **regression** is assumed to be **orthogonal** to the **explanatory variables**, helping find the** best linear unbiased estimator**.

*All images were created with GeoGebra and MATLAB.*