The awesome product

Calculus Level 5

P = n = 1 n e 11 n n + 11 \large{P=\prod _{ n=1 }^{ \infty }{ \frac { n{ e }^{ \frac { 11 }{ n } } }{ n+11 } } }

If P P denotes the above product, evaluate P \left\lfloor P \right\rfloor .


The answer is 22837737537.

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1 solution

Aareyan Manzoor
Jan 13, 2016

we split the product into 2 parts, let e x p ( x ) = e x , H n = k = 1 n 1 k exp(x)=e^x,H_n=\sum_{k=1}^n \dfrac{1}{k} .the product can be written as lim n k = 1 n k k + 11 k = 1 n e x p ( 11 k ) \lim_{n\rightarrow \infty} \prod_{k=1}^n \dfrac{k}{k+11}\prod_{k=1}^n exp\left(\dfrac{11}{k}\right) The first product is just a basic telescoping series - product . the second one can be evaluated using the rules of exponents . = lim n 11 ! ( n + 1 ) ( n + 2 ) . . . ( n + 11 ) e x p ( k = 1 n 11 n ) = lim n 11 ! ( n + 1 ) ( n + 2 ) . . . ( n + 11 ) e x p ( 11 H n ) =\lim_{n\rightarrow \infty}\dfrac{11!}{(n+1)(n+2)...(n+11)}exp\left(\sum_{k=1}^n \dfrac{11}{n}\right)=\lim_{n\rightarrow \infty}\dfrac{11!}{(n+1)(n+2)...(n+11)}exp\left(11H_n\right) we know as n tends towards infinity, so does the harmonic series! But subtracting the log of n from it is finite! This motivates us to write this as lim n 11 ! e x p ( 11 H n ln ( ( n + 1 ) ( n + 2 ) . . . ( n + 11 ) ) ) ) = lim n 11 ! e x p ( ( H n ln ( n + 1 ) ) + ( H n ln ( n + 2 ) ) + . . . ( H n ln ( n + 11 ) ) ) ) \lim_{n\rightarrow \infty}11!exp\left(11H_n-\ln((n+1)(n+2)...(n+11)))\right)\\=\lim_{n\rightarrow \infty}11!exp\left((H_n-\ln(n+1))+(H_n-\ln(n+2))+...(H_n-\ln(n+11)))\right) Remember lim n ( H n ln ( n ) ) = γ . 5772 \lim_{n\rightarrow\infty}\left(H_n-\ln(n)\right)=\gamma\approx .5772 we can also say lim n ( H n ln ( n + 1 ) ) = lim n ( H n + 1 ln ( n + 1 ) ) = γ \lim_{n\rightarrow\infty} (H_n-\ln(n+1))=\lim_{n\rightarrow\infty} (H_{n+1}-\ln(n+1))=\gamma this is beacause lim n 1 n = 0 \lim_{n\rightarrow\infty} \dfrac{1}{n}=0 so, it becomes 11 ! e x p ( 11 γ ) 22837737537 11!exp\left(11\gamma\right)\approx \boxed{22837737537}

I think you need to explain how you are going from H n ln ( n + k ) H_n-\ln(n+k) for k = 1 , . . 11 k=1,..11 to simply H n ln ( n ) H_n-\ln(n) ... that's the key step.

Otto Bretscher - 5 years, 5 months ago

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but isnt the lim n ln ( n ) = lim n ln ( n + k ) \lim_{n\rightarrow\infty }\ln(n)=\lim_{n\rightarrow\infty }\ln(n+k) ?

Aareyan Manzoor - 5 years, 5 months ago

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Well, both of these limits are infinity, so, that does not prove much...

Otto Bretscher - 5 years, 5 months ago

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@Otto Bretscher hmm.... in another way, lim n ( H n ln ( n + 1 ) ) = lim n ( H n + 1 ln ( n + 1 ) ) = γ \lim_{n\rightarrow\infty} (H_n-\ln(n+1))=\lim_{n\rightarrow\infty} (H_{n+1}-\ln(n+1))=\gamma this is beacause lim n 1 n = 0 \lim_{n\rightarrow\infty} \dfrac{1}{n}=0 Does this serve as a good proof?

Aareyan Manzoor - 5 years, 5 months ago

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@Aareyan Manzoor It helps, yes!

Otto Bretscher - 5 years, 5 months ago

The best way to see this is to observe that the partial product is 11 ! n ! ( n + 11 ) ! n 11 e 11 ( H n ln n ) = 11 ! n 11 ( n + 1 ) ( n + 2 ) ( n + 11 ) e 11 ( H n ln n ) \frac{11! n!}{(n+11)!} n^{11} e^{11(H_n - \ln n)} \; = \; 11! \frac{n^{11}}{(n+1)(n+2)\cdots(n+11)} e^{11(H_n - \ln n)} The fraction, being equal to r = 1 11 ( 1 + r n ) 1 \prod_{r=1}^{11}\big(1 + \tfrac{r}{n}\big)^{-1} certainly tends to 1 1 as n n\to\infty , and H n ln n γ H_n - \ln n \,\to\, \gamma as n n \to \infty . Hence the result.

Mark Hennings - 5 years, 5 months ago

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Yes, that's a good explanation! I realized that Aareyan's solution wasn't quite complete, but I did not want to bother him any longer ;)

Otto Bretscher - 5 years, 5 months ago

i did exactly the same thing. i did not get the correct answer. very strange may be because i have taken euler maschoroni constant to be 0.57721.

Srikanth Tupurani - 2 years, 2 months ago

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Not really. The answer contains 11 11 significant tigures, and you are using a value of γ \gamma with only 5 5 SF.

Mark Hennings - 2 years, 2 months ago

exactly. i missed it. thanks.

Srikanth Tupurani - 2 years, 2 months ago

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