# The exponential function

- Author:
- Ilic Ferretti

NEEDING TO BRUSH UP THE EXPONENTS
We have seen that if a quantity changes following an exponential behaviour with respect of a variable , it can expressed by something like:
where is the starting amount of (when ) and if the constant factor by which changes.
We define
Before trying to handle these objects, we need to understand what it means having a power whose exponent can assume means
Now let's see what happens when using some unusual exponents.
(if is NOT zero).
As a matter of fact we have that , and this tells us that:
where in the last step we simplified all elements both in the numerator and in the denominator: as there is the same number of elements in the numerator and in the denominator, in both parts of the fractions we cancel out everything and we are left with 1.
.
as the result of a division like . Considering the calculations we have that:
When simplifying the elements, we have three elements left in the denominator, because 5 exceeds 2 by 3.
This works exactly in the same way of
In this case the number of elements in the dividend exceeds the one in the divisor, so there are some elements left in the
.
To obtain that, . If the raise to the second we get:
This means that as
Please note that is the cube root of , as
as
In general we have that
The denominator of the exponent is the index of the root, where the numerator is the usual exponent.
with being ?
The answer is not easy, so let's help ourselves with a practical application of exponential functions. We will do it studying the behaviour of waterlily, the nice flower you can see below.
In the animation below we will discover that the behaviour of this plant, as supposed in the problem, can be described by an exponential function. We will draw the function giving to the exponent the values we have studied until now. After the animation we will try to go further introducing irrational exponents.

**exponential function**any function where the independent variable appears as exponent, such as**any**real number. To do this, let's review the meaning of some particular powers we already know. REVIEWING POWERS WITH "SPECIAL" EXPONENTS We all know what a power is; for example**ZERO EXPONENT**Using what we know about**the properties of powers**we can obtain that**can be considered the result of a division like****NEGATIVE EXPONENT (exponent is an**Reasoning in a similar way we can obtain that*integer*number)**Once more we use the property of division with like bases**, and we consider*numerator*(i.e. the*upper*side of the fraction). When the exponent is negative we simply have that there are more elements in the divisor, and therefore some elements remain in the*denominator*, i.e. in the*lower*side of the fraction. Please note that in general a negative exponent is equivalent of taking the reciprocal of the base, so**FRACTIONARY EXPONENT (exponent is a**Now we will prove that*rational*number)**we must remember the definition of "**: this is the number which, raised to the second, gives us*square root of*"**behaves exactly as the square root of** , and therefore they are the same number. Obviously this works for*any*fractionary exponent, so we have that**IRRATIONAL EXPONENT (exponent is a**As we want to consider*real*number): ???*any*number, we must ask ourselves what does an*irrational*exponent means. For example, what's the meaning ofWHAT NEXT?
In the example of waterlilies we introduced an exponential function evaluating it for:
weeks?
The question sounds a little bit weird, and indeed it is.
Nonetheless we will try to find an answer in next animation.

**natural exponents**, i.e. after a given number of weeks**negative exponents**, which proved to reproduce the behaviour of n weeks*ago***fractionary (rational) numbers**, which proved to fit into the curve when evaluating the amount after a non integer number of weeks.

**Yet, we haven't evaluated our function in all points were****(or****, in our example) is an irrational number**. For instance what will the extensions of waterlilies be afterA BRIEF PREVIEW OF LIMIT CONCEPT
Let's summarize what we have seen:
The preceding expression is formally expressed as follows:
"the limit of , as approaches , is equal to "
, which can be translated in more common words as:
"the closer [the approximation of] gets to , the closer gets to "
NOTE: we used the symbol , and not simply , as it is intended that is approaching passing ) which are its rational approximations: then , then
We can use the same symbols describe the behaviour of the function in the left side of the graph: the greater and negative it gets the (this is expressed saying that ), the more the power approaches to .
This is read "
Let's now see the general features common to all exponential function like the one we studied in the case of waterlilies.
BASE , but obviously we could have taken another number as base. The first important thing we must take into account, however, is that wil range between as exponent, we must exclude functions with negative bases.
We exclude also , because would be a rather boring functions resulting always (and giving us many troubles when ), so we are not interested in studying it.
This limitation will be present also with logarithms, which are powers seen from a different point of view.
EXPONENTIAL FUNCTION NEVER GETS NEGATIVE
As a direct consequence of the requirement described in previous paragraph, we have that is always a positive number, and therefore the exponential function NEVER gets on or below the axis. You can see this in next image.

**any irrational number con be approximated as precisely as we want with a rational quantity**. For instance**as long as we have a rational number, we know how to consider it as an exponent**, so we get an approximation for the value we are looking for. For instance(ok, you probably may find it not very nice, but we do know how to calculate it!) **to get a better approximation, we simply need to consider more decimals**- so
**we can "virtually" define**.as the number obtained considering as many decimals of as we need

*a limit*: we defined the value of a power with an irrational exponent as the number we obtain going*nearer and nearer*to that irrational value (without reaching it actually, as we do not know how to calculate it). The trick is that we get nearer to it (we*approach it*, as it is said using technical terms)**considering rational exponents, whose power we can calculate with no difficulty**. We can represent this process of*approaching*the irrational value using the symbol of*limits,*which we will meet much later when studying analysis:*by a discrete set of values*, that is not all possible values but only*some*of them (which we can call*tends to a negative infinite quantity*, represented by*the limit of* when approaches negative infinity is zero" GENERAL FEATURES OF EXPONENTIAL FUNCTION__MUST__BE GREATER THAN ZERO In our example the base of the power was**WE CANNOT CONSIDER AN EXPONENTIAL WITH NEGATIVE OR ZERO BASE, therefore it must be**. As a matter of fact, as the exponent*any*real number,**the exponent will assume also values like** , etc..., which are interpreted as roots with even index. We know that such roots must have non negative radicand, so if we want to evaluate an exponential function for*any*value ofBEHAVIOUR CHANGES IF OR
In the waterlilies example we condidered and exponential function with . Its behaviour is similar when assumes many different values, . We see this feature in the next animation.

**but it is not always the same for any value of**A functions like the exponential with base greater than is called , the more increases the (i.e the function's result). Another example of increasing function is a line with positive angular coefficient.
A functions like the exponential with base less than is called , the more decreases the (i.e the function's result). Another example of increasing function is a line with negative angular coefficient.
Finally please note that for the exponential function taking the reciprocal of the base has the same effect of changing the sign of the exponent, that is: the same effect of evaluating the function for instead of . For this reason two exponential functions having reciprocal bases are axis: considering the first one for a given point gives the same result as considering the other one for (that is the symmetrical of with respect of the y axis). You can verify this feature drawing point by point two such functions, like the ones used in previous animation or and .

*("funzione monotona crescente", in Italian), because in***increasing function***any*of its parts you have that the greater gets the*("funzione monotona decrescente", in Italian), because in***decreasing function***any*of its parts you have that the greater gets the**symmetrical**with respect of the