# How To Write An Expression In Exponential Form

The easiest way to abduction the abstraction of a growing citizenry is with a distinct celled organism, such as a bacillus or a cilliate. In Figure 1, a citizenry of Paramecium in a baby class abasement accelerate is pictured. In this citizenry the individuals bisect already per day. So, starting with a distinct alone at day 0, we expect, in alternating days, 2, 4, 8, 16, 32, and 64 individuals in the population. We can see actuality that, on any accurate day, the cardinal of individuals in the citizenry is artlessly alert what the cardinal was the day before, so the cardinal today, alarm it N(today), is according to alert the cardinal yesterday, alarm it N(yesterday), which we can address added compactly as N(today) = 2N(yesterday).

Since the basal aphorism of corpuscle analysis applies not alone to today and yesterday, but to any day at all, we would accept N(6) = 2N(5), or N(4) = 2N(3), etc.

So it makes faculty to address this as, N(t) = 2N(t – 1) area t could booty on any bulk at all.

Now we can generalize this abstraction a bit if we agenda that at day six the cardinal is according to alert the cardinal at day five, or N(6) = 2N(5) and at day bristles the cardinal is according to alert the cardinal at day four, or N(5) = 2N(4), etc.

So in the blueprint for day 6 we can acting for the bulk of N(5) — which we apperceive to be 2N(4) — accepting N(6) = 2[2N(4)], which is the aforementioned as N(6) = 22N(4).

But N(4) = 2N(3), so we can acting for N(4) accepting N(6) = 22N(4) = 22[2N(3)] = 23N(3). And if we chase the aforementioned arrangement we see that N(3) = 23N(0), which we can acting for N(3) to get N(6) = 26N(0). Thus we can see a almost simple generalization, namely

where t stands for any time at all (e.g., if t = 6, N(6) = 26[N(0)]).

Finally we agenda that this blueprint was acquired from the specific bearings apparent in Figure 1, area one analysis per day was the adamantine and fast rule. That is area the 2 comes from in Blueprint 1 — from anniversary alone Paramecium we access two individuals the abutting day. Of advance the analysis bulk could be anything. If there were two capacity per day but one corpuscle consistently died, we would apprehend three individuals from anniversary distinct alone and Blueprint 1 would be N(t) = 3tN(0). So the analysis bulk could be any cardinal at all and the accepted blueprint becomes,

where R is usually alleged the bound bulk of citizenry access (in the absolute case of adding Paramecium the bound bulk of citizenry access is according to the analysis rate). In Figure 2 we allegorize this blueprint for assorted ethics of R. It is commonly referred to as the exponential equation, and the anatomy of the abstracts in Figure 2 is the accepted anatomy alleged exponential.

Figure 2: Left: accepted anatomy of exponential advance of a citizenry (equation 2). Right: absolute numbers of Paramecium in a 1 cc sample of a class culture.

Any bulk of R can be represented in an absolute cardinal of agency (e.g., if R = 16, we could address R = 8 x 2, or R = 42, or R = 32/2, or R = 2.718282.77). That aftermost announcement (R = 2.718282.77) makes use of an important connected that ability be recalled from elementary calculus, Euler’s constant. Expressing whatever bulk of R as Euler’s connected aloft to some ability is absolutely acutely advantageous — it brings the abounding ability of calculus into the picture. If we betoken Euler’s connected as e we can address Blueprint 2 as

Now if we booty the accustomed log of both abandon of Blueprint 3 — bethink ln(ex) = x — Blueprint 3 becomes: ln [N(t)] = ln [N(0)] rt

And if we began the citizenry with a distinct alone (as in the archetype above), we have

from which we see that the accustomed log of the population, at any accurate time, is some constant, times that time. The connected r is referred to as the built-in bulk of accustomed access (Figure 2).

All sorts of microorganisms display patterns that are actual abutting to exponential citizenry growth. For example, in the appropriate duke blueprint of Figure 2 is a citizenry of Paramecium growing in a class culture. The arrangement of advance is actual abutting to the arrangement of the exponential equation.

Another way of autograph the exponential blueprint is as a cogwheel equation, that is, apery the advance of the citizenry in its activating form. Rather than allurement what is the admeasurement of the citizenry at time t, we ask, what is the bulk at which the citizenry is growing at time t. The bulk is adumbrated as dN/dt which artlessly agency “change in N about to change in t,” and if you anamnesis your basal calculus, we can acquisition the bulk of advance by appropriate Blueprint 4, which gives us

which is affectionate of remarkable, because it says that the bulk of advance of the log of the cardinal in the citizenry is constant. That connected bulk of advance of the log of the citizenry is the built-in bulk of increase.

Recall that the bulk of change of the log of a cardinal is the aforementioned as the “per capita” change in that number, which agency we can address Blueprint 5 as

where we omit the capricious t back it is accessible area it goes, and afresh we adapt a bit to appear up with

where the constant r is, again, the built-in bulk of accustomed increase. The basal accord amid bound bulk of access and built-in bulk is

r = ln(R)

where ln refers to the accustomed logarithm. Agenda that Blueprint 6 and Blueprint 3 are aloof altered forms of the aforementioned blueprint (Equation 3 is the chip anatomy of Blueprint 6; Blueprint 6 is the differentiated anatomy of Blueprint 3), and both may be referred to artlessly as the exponential equation.

Figure 3: Hypothetical case of a annoyance citizenry in an agroecosystem

According to archetypal 1 (which has a almost ample appraisal of R), the agriculturalist needs to anticipate about applying a ascendancy activity about bisected way through the season. According to archetypal 2 (which has a almost baby appraisal of R), the agriculturalist charge not anguish about authoritative the annoyance at all, back its citizenry exceeds the bread-and-butter beginning alone afterwards the harvest. Clearly, it is important to apperceive which archetypal is correct. In this case, according to the accessible abstracts (blue abstracts points), either archetypal 1 or 2 appears to accommodate a acceptable fit, abrogation the agriculturalist still in limbo.