# function definition algebra

= is algebraic, being the solution to, Moreover, the nth root of any polynomial ∀ x ∈ A , ∃ y ∈ B ∣ ( x , y ) ∈ G {\displaystyle \forall x\in A,\exists y\in B\mid (x,y)\i… … 1. ≤ Let's examine this: Given the function f (x) as defined above, evaluate the function at the following values: x = –1, x = 3, and x = 1. Another way of looking at it is that we are asking what the $$y$$ value for a given $$x$$ is. x On the other hand, relation #2 has TWO distinct y values 'a' and 'c' for the same x value of '5' . Therefore, the list of second components (i.e. In mathematics, an algebraic function is a function that can be defined Example of One to One Function So, we replaced the $$y$$ with the notation $$f\left( x \right)$$. A function is a relationship between two quantities in which one quantity depends on the other. The only difference is the function notation. Again, let’s plug in a couple of values of $$x$$ and solve for $$y$$ to see what happens. The existence of an algebraic function is then guaranteed by the implicit function theorem. When we determine which inequality the number satisfies we use the equation associated with that inequality. To determine if we will we’ll need to set the denominator equal to zero and solve. A function may be thought of as a rule which takes each member x of a set and assigns, or maps it to the same value y known at its image.. x → Function → y. As this one shows we don’t need to just have numbers in the parenthesis. Evaluation is really quite simple. 3 We are much more interested here in determining the domains of functions. All the $$x$$’s on the left will get replaced with $$t + 1$$. This is a function! f Note that we can have values of $$x$$ that will yield a single $$y$$ as we’ve seen above, but that doesn’t matter. So, when there is something other than the variable inside the parenthesis we are really asking what the value of the function is for that particular quantity. ⁡ Think of an algebraic function as a machine, where real numbers go in, mathematical operations occur, and other numbers come out. ( x the second number from each ordered pair). 3 p = Which half of the function you use depends on what the value of x is. = This function assigns the value 4 in the range to the number −2 in the domain. We will come back and discuss this in more detail towards the end of this section, however at this point just remember that we can’t divide by zero and if we want real numbers out of the equation we can’t take the square root of a negative number. We’ve actually already seen an example of a piecewise function even if we didn’t call it a function (or a piecewise function) at the time. x You can tell by tracing from each x to each y.There is only one y for each x; there is only one arrow coming from each x.: Ha! Then by the argument principle. Now, to do each of these evaluations the first thing that we need to do is determine which inequality the number satisfies, and it will only satisfy a single inequality. As a polynomial equation of degree n has up to n roots (and exactly n roots over an algebraically closed field, such as the complex numbers), a polynomial equation does not implicitly define a single function, but up to n {\displaystyle y=\pm {\sqrt {1-x^{2}}}.\,}. that are polynomial over a ring R are considered, and one then talks about "functions algebraic over R". That won’t change how the evaluation works. A piecewise function is nothing more than a function that is broken into pieces and which piece you use depends upon value of $$x$$. The input of 2 goes into the g function. Function definition A technical definition of a function is: a relation from a set of inputs to a set of possible outputs where each input is related to exactly one output. Let’s take a look at some more examples. In this problem, we take the input, or 7, multiply it by 2 and then subtract 1. Here are the evaluations. For the function f + g, f - g, f.g, the domains are defined as the inrersection of the domains of f and g For f/g , the domains is the intersection of the domains of f and g except for the points where g(x) = 0 Recall the mathematical definition of absolute value. First, we need to get a couple of definitions out of the way. In that example we constructed a set of ordered pairs we used to sketch the graph of $$y = {\left( {x - 1} \right)^2} - 4$$. Let’s take the function we were looking at above. what goes into the function is put inside parentheses after the name of the function: So f(x) shows us the function is called "f", and "x" goes in. Now, let’s get a little more complicated, or at least they appear to be more complicated. A function is an equation for which any $$x$$ that can be plugged into the equation will yield exactly one $$y$$ out of the equation. Recall, that from the previous section this is the equation of a circle. ) The denominator (bottom) of a fraction cannot be zero 2. The list of second components associated with 6 is then : 10, -4. Illustrated definition of Function: A special relationship where each input has a single output. If you keep that in mind you may find that dealing with function notation becomes a little easier. cos This will happen on occasion. At this point, that means that we need to avoid division by zero and taking square roots of negative numbers. for each value of x, then x is also a solution of this equation for each value of y. 1 = So, we will get division by zero if we plug in $$x = - 5$$ or $$x = 2$$. that is continuous in its domain and satisfies a polynomial equation. In Common Core math, eighth grade is the first time students meet the term function. Definition Of One To One Function. In this case there are no variables. Don’t get excited about the fact that the previous two evaluations were the same value. We introduce function notation and work several examples illustrating how it works. In other words, the denominator won’t ever be zero. Now, remember that we’re solving for $$y$$ and so that means that in the first and last case above we will actually get two different $$y$$ values out of the $$x$$ and so this equation is NOT a function. Here is $$f\left( 4 \right)$$. Since there aren’t any variables it just means that we don’t actually plug in anything and we get the following. This determines y, except only up to an overall sign; accordingly, it has two branches: Likewise, we will only get a single value if we add 1 onto a number. Note as well that the value of $$y$$ will probably be different for each value of $$x$$, although it doesn’t have to be. A critical point is a point where the number of distinct zeros is smaller than the degree of p, and this occurs only where the highest degree term of p vanishes, and where the discriminant vanishes. In particular, the argument principle can be used to show that any algebraic function is in fact an analytic function, at least in the multiple-valued sense. Note that the fact that if we’d chosen -7 or 0 from the set of first components there is only one number in the list of second components associated with each. . There are of course many more relations that we could form from the list of ordered pairs above, but we just wanted to list a few possible relations to give some examples. More About One to One Function. So, in this case there are no square roots so we don’t need to worry about the square root of a negative number. Before we give the “working” definition of a function we need to point out that this is NOT the actual definition of a function, that is given above. Instead, it is correct, though long-winded, to write "let $$f\colon \mathbb {R} \to \mathbb {R}$$ be the function defined by the equation f(x) = x , valid for all real values of x ". ) An algebraic function in m variables is similarly defined as a function The ideas surrounding algebraic functions go back at least as far as René Descartes. In this case we’ve got a fraction, but notice that the denominator will never be zero for any real number since x2 is guaranteed to be positive or zero and adding 4 onto this will mean that the denominator is always at least 4. Note that in this case this is pretty much the same thing as our original function, except this time we’re using $$t$$ as a variable. Hence any polynomial relation p(y, x) = 0 is guaranteed to have at least one solution (and in general a number of solutions not exceeding the degree of p in y) for y at each point x, provided we allow y to assume complex as well as real values. 3 However, not every function has an inverse. This one is going to work a little differently from the previous part. y Evaluating a function is really nothing more than asking what its value is for specific values of $$x$$. ( However, before we actually give the definition of a function let’s see if we can get a handle on just what a relation is. Now, when we say the value of the function we are really asking what the value of the equation is for that particular value of $$x$$. ( This tends to imply that not all $$x$$’s can be plugged into an equation and this is in fact correct. y y y So, in the absolute value example we will use the top piece if $$x$$ is positive or zero and we will use the bottom piece if $$x$$ is negative. x > With the exception of the $$x$$ this is identical to $$f\left( {t + 1} \right)$$ and so it works exactly the same way. ( Quite often algebraic functions are algebraic expressions using a finite number of terms, involving only the algebraic operations addition, subtraction, multiplication, division, and raising to a fractional power. ln y There is however a possibility that we’ll have a division by zero error. Let’s take a look at evaluating a more complicated piecewise function. Now, if we multiply a number by 5 we will get a single value from the multiplication. That isn’t a problem. Note that we don’t care that -3 is the second component of a second ordered par in the relation. When defining a function with domain and codomain , it is common to denote it by . , Let’s take care of the square root first since this will probably put the largest restriction on the values of $$x$$. . Therefore, it seems plausible that based on the operations involved with plugging $$x$$ into the equation that we will only get a single value of The most commonly used notation is functional notation, which defines the function using an equation that gives the names of the function and the argument explicitly. x We looked at a single value from the set of first components for our quick example here but the result will be the same for all the other choices. x For the final evaluation in this example the number satisfies the bottom inequality and so we’ll use the bottom equation for the evaluation. p = We then add 1 onto this, but again, this will yield a single value. Here is the list of first and second components, ${1^{{\mbox{st}}}}{\mbox{ components : }}\left\{ {6, - 7,0} \right\}\hspace{0.25in}\hspace{0.25in}{2^{{\mbox{nd}}}}{\mbox{ components : }}\left\{ {10,3,4, - 4} \right\}$. Another way of combining functions is to form the composition of one with another function.. On the other hand, it’s often quite easy to show that an equation isn’t a function. y Note that there is nothing special about the $$f$$ we used here. However, let’s go back and look at the ones that we did plug in. Now the second one. From an algebraic perspective, complex numbers enter quite naturally into the study of algebraic functions. However, all the other values of $$x$$ will work since they don’t give division by zero. That is perfectly acceptable. The relation from the second example for instance was just a set of ordered pairs we wrote down for the example and didn’t come from any equation. A close analysis of the properties of the function elements fi near the critical points can be used to show that the monodromy cover is ramified over the critical points (and possibly the point at infinity). What is important is the “$$\left( x \right)$$” part. And we usually see what a function does with the input: f(x) = x 2 shows us that function "f" takes "x" and squares it. ) {\displaystyle y={\sqrt[{n}]{p(x)}}} Definition: A function is a relation from a domain set to a range set, where each element of the domain set is related to exactly one element of the range set. the first number from each ordered pair) and second components (i.e. x This is simply a good “working definition” of a function that ties things to the kinds of functions that we will be working with in this course. . This is read as “f of $$x$$”. Make sure that you deal with the negative signs properly here. The informal definition of an algebraic function provides a number of clues about their properties. This one is pretty much the same as the previous part with one exception that we’ll touch on when we reach that point. The number under a square root sign must be positive in this section exp So, this equation is not a function. This is also an example of a piecewise function. First, note that any polynomial function Now, let’s see if we have any division by zero problems. In addition, we introduce piecewise functions in this section. This is a function. In many places where we will be doing this in later sections there will be $$x$$’s here and so you will need to get used to seeing that. {\displaystyle a_{i}(x)} x However, as we saw with the four relations we gave prior to the definition of a function and the relation we used in Example 1 we often get the relations from some equation. For $$f\left( 3 \right)$$ we will use the function $$f\left( x \right)$$ and for $$g\left( 3 \right)$$ we will use $$g\left( x \right)$$. In this final part we’ve got both a square root and division by zero to worry about. The inverse is the algebraic "function" So, since we would get a complex number out of this we can’t plug -10 into this function. the list of values from the set of second components) associated with 2 is exactly one number, -3. Now, let’s think a little bit about what we were doing with the evaluations. x A function is a relation for which each value from the set the first components of the ordered pairs is associated with exactly one value from the set of second components of the ordered pair. = is an algebraic function, since it is simply the solution y to the equation, More generally, any rational function However, strictly speaking, it is an abuse of notation to write "let $$f\colon \mathbb {R} \to \mathbb {R}$$ be the function f(x) = x ", since f(x) and x should both be understood as the value of f at x, rather than the function itself. Math Insight. x x The idea of the composition of f with g (denoted f o g) is illustrated in the following diagram.Note: Verbally f o g is said as "f of g": The following diagram evaluates (f o g)(2).. We also give a “working definition” of a function to help understand just what a function is. Now, we can actually plug in any value of $$x$$ into the denominator, however, since we’ve got the square root in the numerator we’ll have to make sure that all $$x$$’s satisfy the inequality above to avoid problems. Again, don’t forget that this isn’t multiplication! This one works exactly the same as the previous part did. = Since relation #1 has ONLY ONE y value for each x value, this relation is a function. If even one value of $$x$$ yields more than one value of $$y$$ upon solving the equation will not be a function. One more evaluation and this time we’ll use the other function. Note that we did mean to use equation in the definitions above instead of functions. In order to officially prove that this is a function we need to show that this will work no matter which value of $$x$$ we plug into the equation. ) . , Here are the ordered pairs that we used. no square root of negative numbers) we’ll need to require that. Function, in mathematics, an expression, rule, or law that defines a relationship between one variable (the independent variable) and another variable (the dependent variable). Note that, away from the critical points, we have, since the fi are by definition the distinct zeros of p. The monodromy group acts by permuting the factors, and thus forms the monodromy representation of the Galois group of p. (The monodromy action on the universal covering space is related but different notion in the theory of Riemann surfaces.). x A compact phrasing is "let $$f\colon \mathbb {R} \to \mathbb {R}$$ with f(x) = x ," where the redundant "be the function" is omitted and, by convention, "for all $$x$$ in the domain of $$f$$" is understood. ) Any number can go into a function as lon… For each $$x$$, upon plugging in, we first multiplied the $$x$$ by 5 and then added 1 onto it. ) We could just have easily used any of the following. If transcendental numbers occur in the coefficients the function is, in general, not algebraic, but it is algebraic over the field generated by these coefficients. ) We now need to look at this in a little more detail. ( ( is an algebraic function, solving the equation, Surprisingly, the inverse function of an algebraic function is an algebraic function. Let’s start this off by plugging in some values of $$x$$ and see what happens. In this section we will formally define relations and functions. which solves a polynomial equation in m + 1 variables: It is normally assumed that p should be an irreducible polynomial. From the definition the domain is the set of all $$x$$’s that we can plug into a function and get back a real number. So, hopefully you have at least a feeling for what the definition of a function is telling us. functions, sometimes also called branches. Thus, a function f should be distinguished from its value f(x0) at the value x0 in its domain. The informal definition of an algebraic function provides a number of clues about their properties. ( Hopefully these examples have given you a better feel for what a function actually is. When we square a number there will only be one possible value. We will have some simplification to do as well after the substitution. Indeed, interchanging the roles of x and y and gathering terms. Γ ± A function […] Regardless of the choice of first components there will be exactly one second component associated with it. In this case the number, 1, satisfies the middle inequality and so we’ll use the middle equation for the evaluation. For example, the function could be defined by the formula with domain D being the real numbers and the range R being the non-negative real numbers. {\displaystyle y^{2}+x^{2}=1.\,} Now, go back up to the relation and find every ordered pair in which this number is the first component and list all the second components from those ordered pairs. Function notation will be used heavily throughout most of the remaining chapters in this course and so it is important to understand it. At this stage of the game it can be pretty difficult to actually show that an equation is a function so we’ll mostly talk our way through it. It is very important to note that $$f\left( x \right)$$ is really nothing more than a really fancy way of writing $$y$$. So the output for this function with an input of 7 is 13. ( In order to really get a feel for what the definition of a function is telling us we should probably also check out an example of a relation that is not a function. ) {\displaystyle f(x)=\cos(\arcsin(x))={\sqrt {1-x^{2}}}} Before starting the evaluations here let’s notice that we’re using different letters for the function and variable than the ones that we’ve used to this point. In this case that means that we plug in $$t$$ for all the $$x$$’s. Before we do that however we need a quick definition taken care of. Before we examine this a little more note that we used the phrase “$$x$$ that can be plugged into” in the definition. 4 First, we squared the value of $$x$$ that we plugged in. Definition of Limit of a Function Cauchy and Heine Definitions of Limit Let $$f\left( x \right)$$ be a function that is defined on an open interval $$X$$ containing $$x = a$$. Now, at this point you are probably asking just why we care about relations and that is a good question. To gain an intuitive understanding, it may be helpful to regard algebraic functions as functions which can be formed by the usual algebraic operations: addition, multiplication, division, and taking an nth root.