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Real numbers · Sequences

Exponential Sequences. The Number e

Before you start, make sure you know: Simple and compound interest on money (High School Year 1 (semesters 1 and 2))
Before you start, make sure you know: Algebraic fractions (High School Year 1 (semesters 1 and 2))

If you put your savings in a bank and they pay you interest more and more times a year (every month, every day, every second…), how much more can you earn? The answer has a very specific ceiling, and that ceiling is one of the most important numbers in mathematics: the number $e$.

The Sequence That Defines the Number $e$

Calculate $\left(1+\dfrac{1}{n}\right)^n$ for larger and larger $n$: with $n=1$ it gives $2$; with $n=10$, $2.5937\ldots$; with $n=100$, $2.7048\ldots$; with $n=10{,}000$, $2.71814\ldots$

Graph of the sequence (1+1/n)^n: the points rise from the axis toward the dashed asymptote of the number e, with ever smaller jumps, without exceeding it. e ≈ 2.71828… (1+1/n)^n grows and approaches e, but never exceeds it

Notice how the sequence approaches $e$ without exceeding it, no matter how large $n$ is:

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Calculate the limit of the following exponential sequence: $a_n = \left( 1 + \frac{1}{n} \right)^{3n}$

Calculate the limit of the following exponential sequence:

$a_n = \left( 1 + \frac{1}{n} \right)^{3n}$

Solve
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Determine the value that the sequence $b_n$ tends to as $n \to \infty$: $b_n = \left( 1 + \frac{1}{2n} \right)^{2n}$

Determine the value that the sequence $b_n$ tends to as $n \to \infty$:

$b_n = \left( 1 + \frac{1}{2n} \right)^{2n}$

Solve
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Calculate the limit of the sequence with general term: $e_n = \left( 1 - \frac{1}{n} \right)^n$

Calculate the limit of the sequence with general term:

$e_n = \left( 1 - \frac{1}{n} \right)^n$

Solve

The General Formula for $1^{\infty}$

The same idea repeats with many bases and exponents. There is a formula that turns any $1^{\infty}$ into a simpler limit: exponent times (base minus 1).

Diagram of the indeterminate form 1 raised to infinity and its resolution with a power of base e. If the base → 1 and the exponent → ∞: lim (base)^(exponent) = e^L L = lim exponent · (base − 1) exponent · (base − 1)(calculated first) that number L
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Find the limit of the sequence formed by the quotient of polynomials raised to $n$: $c_n = \left( \frac{n+1}{n} \right)^n$

Find the limit of the sequence formed by the quotient of polynomials raised to $n$:

$c_n = \left( \frac{n+1}{n} \right)^n$

Solve
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Determine the limit of the following sequence, given that it presents an indeterminate form of the type $1^\infty$: $f_n = \left( 1 + \frac{4}{n} \right)^n$

Determine the limit of the following sequence, given that it presents an indeterminate form of the type $1^\infty$:

$f_n = \left( 1 + \frac{4}{n} \right)^n$

Solve
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Calculate the limit of the following sequence, resolving the corresponding indeterminate form: $a_n = \left( \frac{n+3}{n-1} \right)^{2n}$

Calculate the limit of the following sequence, resolving the corresponding indeterminate form:

$a_n = \left( \frac{n+3}{n-1} \right)^{2n}$

Solve
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Determine the limit of the following sequence by resolving the indeterminate form of the type $1^\infty$: $b_n = \left( \frac{2n+5}{2n+1} \right)^{3n-2}$

Determine the limit of the following sequence by resolving the indeterminate form of the type $1^\infty$:

$b_n = \left( \frac{2n+5}{2n+1} \right)^{3n-2}$

Solve

A More Complicated Algebraic Exponent

Sometimes the exponent is neither $n$ nor a simple multiple of it, but an algebraic fraction. The procedure is identical: calculate $b_n(a_n-1)$ and solve that limit with the usual techniques (comparing degrees, simplifying algebraic fractions…).

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Find the limit of the exponential sequence whose exponent is $n+5$: $g_n = \left( 1 + \frac{1}{n} \right)^{n+5}$

Find the limit of the exponential sequence whose exponent is $n+5$:

$g_n = \left( 1 + \frac{1}{n} \right)^{n+5}$

Solve
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Find the limit of the sequence combining second-degree polynomials in the base: $c_n = \left( \frac{n^2+2n}{n^2-1} \right)^{5n}$

Find the limit of the sequence combining second-degree polynomials in the base:

$c_n = \left( \frac{n^2+2n}{n^2-1} \right)^{5n}$

Solve
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Calculate the limit of the following sequence, which features an algebraic exponent in fractional form: $d_n = \left( \frac{n-2}{n+4} \right)^{\frac{n^2+1}{n}}$

Calculate the limit of the following sequence, which features an algebraic exponent in fractional form:

$d_n = \left( \frac{n-2}{n+4} \right)^{\frac{n^2+1}{n}}$

Solve
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Calculate the limit of the exponential sequence formed by second-degree polynomials in both the base and the exponent:…

Calculate the limit of the exponential sequence formed by second-degree polynomials in both the base and the exponent:

$a_n = \left( \frac{2n^2-3n+1}{2n^2+n-5} \right)^{\frac{n^2+3}{n}}$

Solve

The Number $e$ in Continuous Compound Interest

If you compound a principal $C$ at a nominal annual interest rate $r$, $n$ times per year, after one year you have $C\left(1+\dfrac{r}{n}\right)^n$. When $n\to\infty$ (continuous compounding, instant by instant), the number $e$ appears.

Diagram showing the move from annual to monthly to continuous compounding, with limit factor e raised to r. annual monthly continuous (1+r/n)^n → e^r as n → ∞ the more times per year it is compounded, the closer it gets to the limit e^r
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An amount of $100\,000$ pesos is deposited in a financial institution at an annual interest rate of $4\%$. The formula for the final capital after $n$ years with…

An amount of $100\,000$ pesos is deposited in a financial institution at an annual interest rate of $4\%$. The formula for the final capital after $n$ years with continuous compounding is given by the sequence $C_n = 100\,000 \times e^{0.04n}$. Calculate the accumulated capital after $10$ years and compare it with the capital obtained if the compounding were annual, whose expression is $C'_n = 100\,000 \times (1,04)^n$.

Solve
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An amount of $200\,000$ pesos is invested at a nominal annual interest rate of $5\%$. The sequence describing the final capital if interest is compounded $n$ times a…

An amount of $200\,000$ pesos is invested at a nominal annual interest rate of $5\%$. The sequence describing the final capital if interest is compounded $n$ times a year is $C_n = 200\,000 \left( 1 + \frac{0.05}{n} \right)^n$. Calculate the limit of this sequence as $n \to \infty$ to find the final capital under continuous compounding. Compare this result with the capital obtained if compounded quarterly ($n=4$).

Solve

Exponential Growth and Decay with $e$

Many processes (radioactive decay, cooling, elimination of a medication) are described by $a_n = a_0\,e^{kn}$, with $k>0$ for growth and $k<0$ for decay. Geometric sequences $a_0 r^n$ follow the same idea: if $|r|<1$ they tend to $0$; if $r>1$ they grow without bound.

Graphs of exponential growth and decay depending on the sign of k. decay: k < 0, the value tends to 0 growth: k > 0, the value tends to +∞
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State the limit of the following geometric sequence: $d_n = 5 \times \left( \frac{2}{3} \right)^n$

State the limit of the following geometric sequence:

$d_n = 5 \times \left( \frac{2}{3} \right)^n$

Solve
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The amount of a radioactive isotope in a sample decreases exponentially according to the sequence $M_n = 200 \times e^{-0.1n}$, where $n$ is the number of years elapsed…

The amount of a radioactive isotope in a sample decreases exponentially according to the sequence $M_n = 200 \times e^{-0.1n}$, where $n$ is the number of years elapsed and $M_n$ is measured in grams. How many grams will remain in the sample after $20$ years? What value does the mass tend to in the long term (as $n \to \infty$)?

Solve
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A vehicle loses $15\%$ of its value each year due to depreciation. If its initial price at the dealership is $24000$ pesos, its value after $n$ years is given by the…

A vehicle loses $15\%$ of its value each year due to depreciation. If its initial price at the dealership is $24000$ pesos, its value after $n$ years is given by the sequence $V_n = 24000 \times (0,85)^n$. What will its value be after $5$ years? What happens to its value as the number of years tends to infinity?

n Vₙ ($) 0 2 4 6 8 10 24000 12000
Solve
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In a thermodynamics experiment, the temperature difference between a metallic body and its surroundings decreases exponentially according to the sequence…

In a thermodynamics experiment, the temperature difference between a metallic body and its surroundings decreases exponentially according to the sequence $T_n = 80 \times e^{-0.05n}$, where $n$ is the minutes elapsed since the start of cooling. Calculate the temperature difference after $30$ minutes and determine the limit of the sequence as $n \to \infty$.

Solve

Harder Cases

The same formula solves bases with roots, unknown parameters, or accumulated doses.

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Calculate the limit of the sequence: $h_n = \left( \frac{n}{n+1} \right)^n$

Calculate the limit of the sequence:

$h_n = \left( \frac{n}{n+1} \right)^n$

Solve
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Find the value of the real parameter $a$ such that the following equality involving the number $e$ holds: $\lim_{n \to \infty} \left( \frac{n+a}{n+1} \right)^{2n} = e^6$

Find the value of the real parameter $a$ such that the following equality involving the number $e$ holds:

$\lim_{n \to \infty} \left( \frac{n+a}{n+1} \right)^{2n} = e^6$

Solve
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A patient receives a dose of $50$ mg of a medication every $24$ hours. Given that the body eliminates $40\%$ of the medication present during that time interval, the…

A patient receives a dose of $50$ mg of a medication every $24$ hours. Given that the body eliminates $40\%$ of the medication present during that time interval, the quantity accumulated in the body immediately after the $n$th dose follows the geometric sequence $C_n = 50 + 50 \times 0.6 + 50 \times 0.6^2 + \dots + 50 \times 0.6^{n-1}$. Calculate the long-term accumulated quantity (the limit of the sequence as $n \to \infty$).

n Cₙ (mg) Limit = ? 1 3 5 7 9 50 100
Solve
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Calculate the values of the parameters $a$ and $b$ such that the following sequence has limit $e^{-4}$, given that $a$ is a positive real number:…

Calculate the values of the parameters $a$ and $b$ such that the following sequence has limit $e^{-4}$, given that $a$ is a positive real number:

$\lim_{n \to \infty} \left( \frac{an+3}{2n+1} \right)^{bn} = e^{-4}$

Solve
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Find the limit of the sequence combining square roots in its base and a linear exponent: $c_n = \left( \sqrt{\frac{n+2}{n-1}} \right)^{3n+1}$

Find the limit of the sequence combining square roots in its base and a linear exponent:

$c_n = \left( \sqrt{\frac{n+2}{n-1}} \right)^{3n+1}$

Solve
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The concentration of a pollutant in a lake is naturally reduced by a process that follows the sequence $C_n = C_0 \left( 1 - \frac{2}{n} \right)^n$, where $n$ represents…

The concentration of a pollutant in a lake is naturally reduced by a process that follows the sequence $C_n = C_0 \left( 1 - \frac{2}{n} \right)^n$, where $n$ represents the number of filtering stages applied. If infinitely many filtering stages are applied, prove mathematically to what exact fraction of the initial concentration $C_0$ the pollutant will be reduced.

Solve
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Calculate the limit of the sequence by resolving the indeterminate form generated by the algebraic quotient in the exponent:…

Calculate the limit of the sequence by resolving the indeterminate form generated by the algebraic quotient in the exponent:

$d_n = \left( \frac{n^2+5n-2}{n^2+3n+1} \right)^{\frac{n^3}{n^2+1}}$

Solve

Back to the beginning: if you invest at an annual rate $r$, paying interest continuously does not send your earnings to infinity: your capital is bounded by $C\,e^r$. At $5\%$, the return goes from $5\%$ to a ceiling of $5.13\%$.

Summary

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