Does $(1+\frac{x}{n})^n$ Uniformly Converge to $e^x$ on Different Intervals?

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In summary, the conversation discusses the convergence of the function $f_n(x)=(1+\frac{x}{n})^{n}$ and its comparison with the exponential function $e^{x}$ at different intervals. It is found that $f_n$ does not converge uniformly on either the interval $(-\infty,a)$ or $(a,+\infty)$, as the exponential function grows faster than any polynomial function. This is demonstrated by considering the specific case of $a=0$ and looking at the behavior of the functions as $x$ approaches $-\infty$.
  • #1
evinda
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Hey again! (Blush)
I am looking at the following exercise:
We suppose $f_n(x)=(1+\frac{x}{n})^{n}, x \in \mathbb{R}$.
Check if $f_n$ converges uniformly at the intervals $(-\infty,a)$ and $(a,+\infty)$,where $a$ is a random real number.

$lim_{n \to +\infty} f_n(x)=e^{x}$,so $f(x)=e^{x}$
For the interval $(a,+\infty)$,we have:
$sup_{x>a}|f_n(x)-f(x)|=sup_{x>a}|(1+\frac{x}{n})^{n}-e^x|$
To find the supremum,we have to know if $ (1+\frac{x}{n})^{n}>e^x$ or $(1+\frac{x}{n})^{n}<e^x$ ,right? But how can I find which is greater?

I looked at the solution of the exercise and saw that at the interval $(a,+\infty)$,they take $e^x-(1+\frac{x}{n})^{n}>0$,and at $(-\infty,a)$ $(1+\frac{x}{n})^{n}-e^x>0$.. But why is it like that?How can I know it?? (Thinking)
 
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  • #2
Let us pick $a=0$ and examine the interval $(0,\infty)$, does $f_n$ converge to $\exp$ on this interval uniformly?

If it did it would mean that for $\varepsilon = 1$, there be an $n$ such that,
$$ |\exp(x) - f_n(x)| < 1 \text{ for all }x>0 $$

But given any $n$ the function $f_n$ is a polynomial function of degree $n$, while $\exp$ is the exponential function which goes to $\infty$ faster than any polynomial, no matter how big the degree. Thus, given any $n$ we can find $x_0$ large enough such that $\exp(x_0) > f_n(x_0) + 1$ which contradicts what we need to have uniform convergence.
 
  • #3
ThePerfectHacker said:
Let us pick $a=0$ and examine the interval $(0,\infty)$, does $f_n$ converge to $\exp$ on this interval uniformly?

If it did it would mean that for $\varepsilon = 1$, there be an $n$ such that,
$$ |\exp(x) - f_n(x)| < 1 \text{ for all }x>0 $$

But given any $n$ the function $f_n$ is a polynomial function of degree $n$, while $\exp$ is the exponential function which goes to $\infty$ faster than any polynomial, no matter how big the degree. Thus, given any $n$ we can find $x_0$ large enough such that $\exp(x_0) > f_n(x_0) + 1$ which contradicts what we need to have uniform convergence.

I understand..And how can I check the uniform converge at the interval $(-\infty,a)$?
 
  • #4
evinda said:
I understand..And how can I check the uniform converge at the interval $(-\infty,a)$?

Pick $a=0$ to get an idea of what is going on $(-\infty,0)$. We have a polynomial $p_n(x)$ of degree $n$ and exponential function $\exp(x)$. As $x\to -\infty$ the polynomial $p_n(x)$ will approach either $\pm \infty$ depending on the sign of its highest degree coefficient. While $\exp(x) \to 0$ as $x\to -\infty$. Therefore, $\exp(x)$ and $p_n(x)$ will not stay close to one another but will deviate as $x\to -\infty$.
 
  • #5


Hello! The supremum-inequality is a mathematical concept that states that the supremum of a set of numbers is always greater than or equal to any element in that set. In this case, we are looking at the supremum of the function $f_n(x)$ over the interval $(a,+\infty)$. To find the supremum, we can use the fact that $f_n(x)$ is a monotonically increasing function, meaning that as $n$ increases, so does the value of $f_n(x)$. Therefore, we can take the limit as $n \to +\infty$ to find the supremum of $f_n(x)$ over the given interval. In this case, we can see that the limit is equal to $e^x$, which is the supremum of $f_n(x)$ over the interval $(a,+\infty)$.

As for finding which is greater between $(1+\frac{x}{n})^{n}$ and $e^x$, we can use the fact that $e^x$ can be written as a power series, which converges for all real numbers. Therefore, we can approximate $e^x$ by taking the first few terms of the power series and compare it to $(1+\frac{x}{n})^{n}$. As $n$ increases, we can see that $(1+\frac{x}{n})^{n}$ gets closer and closer to $e^x$, meaning that eventually, $(1+\frac{x}{n})^{n}$ will be greater than $e^x$.

I hope this helps clarify things for you! Let me know if you have any other questions.
 

FAQ: Does $(1+\frac{x}{n})^n$ Uniformly Converge to $e^x$ on Different Intervals?

What is the definition of supremum?

Supremum, also known as the least upper bound, is the smallest real number that is greater than or equal to all elements in a set.

What is the supremum-inequality?

The supremum-inequality is a mathematical inequality that states that the supremum of a set is always greater than or equal to any element in that set.

How is the supremum-inequality used in mathematics?

The supremum-inequality is commonly used in mathematical analysis and is a fundamental concept in real analysis. It is used to prove the existence of upper bounds for sets and to compare the size of different sets.

What is the difference between supremum and maximum?

The supremum of a set is the smallest real number that is greater than or equal to all elements in the set, while the maximum is the largest element in the set. The supremum may or may not be an element of the set, while the maximum must be an element of the set.

Can the supremum of a set be infinite?

Yes, the supremum of a set can be infinite. For example, the supremum of the set of all positive real numbers is infinite because there is no upper bound for this set.

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