The Difference Between Functions And Equations?

1. ScienceNerd36

79
Hello there my fellow complex number comrades,

I was just wondering, what are the key differences between a function and an equation?

2. Office_Shredder

4,487
Staff Emeritus
Depends on how technical you want to be. What you're probably looking for is something like this:

A function is usually described by an equation f(x)=...*insert what f(x) actually is* or through a similar method. The formal definition of a function is a set of pairs {(x,y)|x is in A and y is in B} where f maps A to B, but that's fairly tedious to work with so we use shortcuts like the above equation. The requirements to be a function are that it must map every point in the domain to a point in the range, and can only map each point in the domain to one point in the range. Using the formal definition above, that means for each a in A, there exists b in B such that (a,b) is in the set and for each a in A, there is only one b such that (a,b) is in the set.

An equation is simply something of the form X=Y where x and y are strings of characters that have to make sense, and the statement X=Y implies that X and Y evaluate to the same thing

3. ScienceNerd36

79
So an equation expresses the relationship between two things while a function allows you to deduce information about a domain?

4. Дьявол

365
Actually you can form equation of functions

Ex. f(a)-f(b)=0

5. ScienceNerd36

79
And that's displaying the relationship between two functions.

6. Дьявол

365
Yep, and every function is equation since,

''An equation is a mathematical statement, in symbols, that two things are exactly the same (or equivalent)''

So f(x) is equal to x+5, or f(x)=x+5 or f(x)-x-5=0.

Regards.

7. flatmaster

502
Functions are a subset of all equations. A function is an equation with the restrictions mentioned earlier.

8. CaffeineJunky

26
An equation is simply a statement: "1 = 1" is an equation that is true. Equality itself is a relation on a set. In fact, equality is an example of an equivalence relation--a relation that satisfies certain properties.

A function is a much more abstract mathematical object that acts as a map from one set to another by mapping each element in the domain set to an element in the codomain set. A function is a special case of a relation.

(x^2 + y^2 = 1 | x, y real numbers) is a relation on the R x R, but not a function.

9. Elucidus

286
An equation is a special type of relation called a comparison and is also a logical statement. It also needs a context (oftimes called a "domain" but not in the same sense as the domain of a function) which is usually some sort of Cartesian product of the sets from whence the unbound variables are selected.

The equation

$z^2=x^2+y^2-2xy\cos{\theta}$

is defined for all values x, y, z, and $\theta$ for which it makes sense.

The equation

$P(E) = P(E|F)\cdotP(F)+P(E|F^\prime)\cdotP(F^\prime)$

is defined for all events in a sample space.

A function is an ordered triple $\langle A, B, \Gamma_f\rangle$ where f is a function from the domain A to the codomain B and has graph $\Gamma_f$ that is the set $\{(a,f(a))|a\in A\}$. As mentioned before each argument a in A must have exactly one value b in B.

The major difference is that an equation is logical object, while a function is an ordered triple (a set theoretic object).

Many functions are defined via equations, but that is not necessary, and in fact it has been proven that equations cannot detail all functions (there are only a countable number of equations while there are uncountable functions).

--Elucidus

10. Tac-Tics

810
A function is a rule. It associates with each possible input a single output.

A function's definition often comes in the form of an equation, but it need not. For a polynomial, we use the equation definition: f(x) = x^2 - 1. For piecewise functions, we break the definition up over several equations. For example:

f(x) = sin(x) / x, when x /= 0
f(x) = 1, when x = 0

Other times, the function's rule might just be given orally. For example, the function which maps vectors to their length.

Equations do not always lead to a good function definition. The easiest example is $$x = y^2$$. Because there are two possible y's for each x (a positive one and a negative one). Luckily in this case, we have a notation we use for a very closely related function: $$f(x) = \sqrt{x}$$.

11. ScienceNerd36

79
So taking analytical geometry as an example:

What would be the key difference in the result between using a function to graph a line, such as "f(x)=3x-2", and using the equation of that line, which would be "3x-y-2"?

12. MattC72

6
3x-y-2 doesn't mean anything (In a geometry sense) unless you have an equivalence relation (In this case, an equals sign). So if you had

$$3x-y-2 = 0$$
then naturally you would have
$$y=3x-2$$
and, therefore, the same as before.

But, if you mean plotting
$$f(x,y) = z = 3x-y-2$$
then you will get a 3-D graph (with axis (x,y,z)).

However, back to your original question; I think everyone's answers are a little too complicated. And, from the way you worded your question, it doesn't sound like you need a textbook answer; it sound like you just need a brief description so that you can get your head around it. So here goes:

A function is an instruction to do something to an object. I think of functions as machines. So input is a load of fruit, output is a smoothie. You put something in, you get something (usually different) out.

Equations are a type of function. Put number(s) in, get number(s) out.

So,
$$Set \ of \ Functions \supset Set \ of \ Equations$$

Matt

p.s. If this post makes you want to drink smoothies, I take no responsibility for the havoc it'll cause your insides.

13. Tac-Tics

810
I'm assuming you mean 3x - y - 2 = 0. By itself, 3x - y - 2 is just an expression. You need a verb (usually an equals sign) to make it an equation.

If you graph the function f in the standard way, you do get a line in the plane which is uniquely defined by the equation 3x - y - 2 = 0. The two are very intimately related.

But the graph isn't the function. And an equation isn't a function. And an equation isn't a graph. These are all distinct concepts.

The equation is a statement with two free variables.

A graph is a set of points. In this case, the set of points is given by the set { (x, y) for all x, y where 3x - y - 2 = 0 }.

The function, f, is the rule. Keep in mind that f and y are two completely, totally different things! In the equation, y is a real number, and f is a function! However, when you apply an argument to f, you get a real number out. And so, f(x) is a real number.

In this case (and many others in algebra and calculus), the three are practically identical. But there are examples of each that don't meet the requirements of the other:

A graph of a square represents no function.
A specially defined function such as f(x) = 1, if x is rational and 0 otherwise has no algebraic equation behind it.
You can also have functions between things that aren't the real numbers. For example, the derivative operator. The derivative is a function which acts on real functions. It has no graph. It has no algebraic equation behind it.

14. Elucidus

286
Unfotunately the relationship between them is the reverse.

$x^2+y^2=1$ is an equation but does not describe either variable as a function of the other.

--Elucidus

15. Tac-Tics

810
Also, equivalence relation means something different than what you've used the word for here.

An equivalence relation is any relation (or you can think of it as a binary predicate, LIKE an equals sign) which exhibits the most important properties of equality: reflexivity, symmetry, and transitivity.

16. ScienceNerd36

79
Thank You! I understand now. And I'm going to make a smoothie.

17. MattC72

6
Is this true?

You are right that $x^2+y^2=1$ is not a bijective function, but the 2 variables are functions of each other.

18. Tac-Tics

810
No smoothie, because this is plain false. Some equations lend themselves to functions. Some don't. People keep throwing $$x^2 = y^2$$ as a simple example.

19. MattC72

6
Yes, you are right in your definition of an equivalence relation, as a special type of binary operation. But commonplace notation holds that equivalence relations are characterized by their individual "equals signs". For example;

$$\cong \ and$$~

20. MattC72

6
In this example:
$$x^2 = y^2$$
is this not a function:
$$f: A \to A$$
defined by
$$f:x\to \pm x$$
?