# What are linear combinations and linear operators in differential equations?

In summary, In a differential equations course, the chapter on "linear differential equations: basic theory" discusses the concepts of linear combination and linear operator. A linear combination involves only multiplication by numbers and addition or subtraction, while a linear operator "preserves" operations such as matrix multiplication, differentiation, and integration. These concepts are important in understanding and solving linear differential equations.
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## Homework Statement

I am in a differential equations course currently. The chapter I'm reading is "linear differerntial equations: basic theory". The words linear combination and linear operator are used.

## Homework Equations

L{$$\alpha$$f(x) + $$\beta$$g(x)} = $$\alpha$$L(f(x)) + $$\beta$$L(g(x))

## The Attempt at a Solution

I forgot what the quick and easy definition of "linear combination was. I seem to remember something about "closed under addition and scalar multiplication". Perhaps somebody could help me re-learn this concept. I took Calc III and Linear Algebra and passed with flying colors but time has eaten my brain.

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A linear combination of, say, x1, x2, ..., xn must involve only multiplication by numbers and addition or subtraction: a1x1+ a2x2+ ...+ anxn where every "a" is a number. Anything more complicated than just "multiply by numbers and add", for example x2 or x/y or cos(x) is NOT linear.

A linear operator is an operator that "preserves" the operations: T(ax+ by)= aT(x)+ bT(y) where a and b are numbers. Matrix multiplication of vectors, differentiation of functions and integration of functions are examples of linear operators.

## 1. What is a linear combination?

A linear combination is a mathematical operation that involves multiplying each element in a set of numbers by a constant and then adding the results together. This is often represented using the notation a1x1 + a2x2 + ... + anxn, where a1, a2, ..., an are the constants and x1, x2, ..., xn are the elements being multiplied.

## 2. What is the purpose of a linear combination?

The purpose of a linear combination is to create a new set of numbers by combining existing sets of numbers in a specific way. This allows for the manipulation and analysis of data in various fields, such as mathematics, physics, and economics.

## 3. How is a linear combination used in real-life situations?

A linear combination is used in many real-life situations, such as calculating the average price of a stock portfolio, determining the optimal combination of ingredients in a recipe, and predicting the trajectory of a projectile in physics. It is also commonly used in data analysis and machine learning algorithms.

## 4. What is the difference between a linear combination and a linear transformation?

A linear combination involves combining existing sets of numbers using multiplication and addition, while a linear transformation involves transforming a set of numbers using operations such as rotation, reflection, and scaling. In other words, a linear combination creates a new set of numbers, while a linear transformation changes the properties of an existing set of numbers.

## 5. How is a linear combination related to linear independence?

In linear algebra, a set of vectors is considered linearly independent if no vector in the set can be represented as a linear combination of the other vectors in the set. This means that the vectors are unique and cannot be created through a linear combination of other vectors. Linear independence is important in linear combinations because it determines the number of possible combinations that can be created from a set of vectors.

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