Can you prove the inequality for positive integers?

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In summary, the inequality for positive integers is a mathematical statement that compares two numbers and determines whether one is greater than, less than, or equal to the other. It can be proven using methods such as mathematical induction and contradiction, and it is important in mathematics for performing operations and ensuring accurate results. This concept also has various real-life applications in fields such as economics, physics, and computer science. While there is no specific formula or rule for proving the inequality, there are general principles and methods that can be used in different cases.
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Ackbach
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My apologies for being late this week. Here is this week's POTW:

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Show that for every positive integer $n$,
\[
\left( \frac{2n-1}{e} \right)^{\frac{2n-1}{2}} < 1 \cdot 3 \cdot 5
\cdots (2n-1) < \left( \frac{2n+1}{e} \right)^{\frac{2n+1}{2}}.
\]

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Remember to read the http://www.mathhelpboards.com/showthread.php?772-Problem-of-the-Week-%28POTW%29-Procedure-and-Guidelines to find out how to http://www.mathhelpboards.com/forms.php?do=form&fid=2!
 
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  • #2
Re: Problem Of The Week # 225 - Jul 21, 2016

This was Problem B-2 in the 1996 William Lowell Putnam Mathematical Competition.

Congratulations to Opalg for his correct (of course) solution. He appeared to be on a roll this week, answering every single level of POTW! Here is his solution:

Let $f(x) = x(\ln x - 1)$. Notice that $f(x)$ is the indefinite integral of $\ln x$. Notice also that $f(1) = -1 < 0$; and $f(3) = 3(\ln3-1) > 0$ (because $3>e$ and so $\ln3 > 1$).

Therefore \(\displaystyle \int_1^{2n-1}\ln x\,dx = f(2n-1) - f(1) > f(2n-1) = (2n-1)\bigl(\ln(2n-1) - 1\bigr).\) Now form the upper Riemann sum for this integral, using subintervals of length $2$ on the interval $[1,2n-1].$ Since $\ln x$ is an increasing function, its supremum on each subinterval will be its value at the right-hand endpoint of the subinterval. The upper Riemann sum is therefore $2\bigl(\ln3 + \ln5 +\ldots + \ln(2n-1)\bigr).$ The upper Riemann sum is larger than the integral, so that $$(2n-1)\bigl(\ln(2n-1) - 1\bigr) < \int_1^{2n-1}\ln x\,dx < 2\bigl(\ln3 + \ln5 +\ldots + \ln(2n-1)\bigr).$$ Adding $\ln 1\;(=0)$ makes no difference to the sum, so $\dfrac{2n-1}2\bigl(\ln(2n-1) - 1\bigr) < \ln1 + \ln3 + \ln5 +\ldots + \ln(2n-1).$ Now take the exponential of both sides to get $$\Bigl(\dfrac{2n-1}e\Bigr)^{(2n-1)/2} < 1\cdot 3 \cdot 5 \cdots (2n-1).$$

The proof for the other inequality is essentially the same. This time, use the interval $[3,2n+1].$ Again, divide it into subintervals of length 2, and form the lower Riemann sum for \(\displaystyle \int_3^{2n+1}\ln x\,dx = f(2n+1) - f(3) < f(2n+1) = (2n+1)\bigl(\ln(2n+1) - 1\bigr)\) (because $f(3) > 0$). The lower Riemann sum this time is $2\bigl(\ln1 + \ln3 + \ln5 +\ldots + \ln(2n-1)\bigr).$ It is less than the integral, and so $$2\bigl(\ln1 + \ln3 + \ln5 +\ldots + \ln(2n-1)\bigr) < \int_3^{2n+1}\ln x\,dx < (2n+1)\bigl(\ln(2n+1) - 1\bigr).$$ Therefore $\ln1 +\ln3 + \ln5 +\ldots + \ln(2n-1) < \dfrac{2n+1}2\bigl(\ln(2n+1) - 1\bigr).$ As previously, exponentiate, to get $$1\cdot 3 \cdot 5 \cdots (2n-1) < \Bigl(\dfrac{2n+1}e\Bigr)^{(2n+1)/2}.$$
 

FAQ: Can you prove the inequality for positive integers?

1) Can you explain what the inequality for positive integers is?

The inequality for positive integers is a mathematical statement that compares two numbers and determines whether one is greater than, less than, or equal to the other. For example, 5 > 3 means that 5 is greater than 3, while 4 < 7 means that 4 is less than 7.

2) How do you prove the inequality for positive integers?

There are several methods for proving the inequality for positive integers. One way is to use mathematical induction, which involves proving that the inequality holds for a base case (usually 1 or 0) and then showing that if it holds for any given number, it also holds for the next consecutive number. Another method is to use contradiction, where you assume that the inequality is false and then show that this leads to a contradiction or inconsistency.

3) Why is proving the inequality for positive integers important?

The inequality for positive integers is an important concept in mathematics that allows us to compare numbers and perform operations such as addition, subtraction, multiplication, and division. By proving the inequality, we can ensure that these operations will always produce accurate and meaningful results.

4) Are there any real-life applications of the inequality for positive integers?

Yes, the inequality for positive integers has many real-world applications. For example, it is used in economics to analyze supply and demand, in physics to calculate forces and acceleration, and in computer science to design efficient algorithms.

5) Is there a specific formula or rule for proving the inequality for positive integers?

There is no specific formula or rule for proving the inequality for positive integers, as it depends on the specific numbers and situation. However, there are some general principles and methods, such as mathematical induction and contradiction, that can be applied to prove the inequality in different cases.

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