Hot Integral - 28

Calculus Level 5

lim n 1 Γ ( n + 1 ) 0 0 0 0 1 ( i = 1 n x i ) ( i = 1 n x i + 1 x i ) 2 d x 1 d x 2 d x n \displaystyle \lim_{n \to \infty}\frac{1}{\Gamma(n+1)} \int\limits_{0}^{\infty} \int\limits_{0}^{\infty} \int\limits_{0}^{\infty} \dots \int\limits_{0}^{\infty} \frac{1}{(\prod_{i=1}^{n} x_i)(\sum_{i=1}^{n} x_i +\frac{1}{x_i})^2}\, dx_1 dx_2 \cdots dx_n

The above expression can be expressed as A B e C γ \large \frac{A}{Be^{C\gamma}}

Write the answer as the concatenation of the sum A B C + C B A ABC+CBA of the integers A , B , C A,B,C . For example, if you think that A = 1 , B = 2 , C = 3 A=1,B=2,C=3 , type the answer as 123 + 321 = 444 123+321=\boxed{444} .

Moreover , if you think that the above expression is not possible and you think that limit does not exist, enter your answer as 888.


This is a part of Hot Integrals


The answer is 343.

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1 solution

Mark Hennings
Aug 21, 2016

This integral is, in the notation of this paper on Ising susceptibility integrals, equal to 1 4 C n \tfrac14C_n , and this paper proves that lim n C n = 2 e 2 γ \lim_{n\to\infty} C_n = 2e^{-2\gamma} . Thus the limit of the integral in this question is 1 2 e 2 γ \tfrac12e^{-2\gamma} , making A = 1 , B = 2 , C = 2 A=1,B=2,C=2 , and hence the answer is 122 + 221 = 343 122 + 221 = \boxed{343} .

Can you explain me how to.calcuate this integral. It is not clear from.the paper.he is bounding the expression.

Srikanth Tupurani - 2 years, 5 months ago

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The result is stated as a conjecture on page 5, but proved by Theorem 2 on page 10. The result was initially conjectured from computer testing. They do not evaluate the integral, just the limit.

Mark Hennings - 2 years, 5 months ago

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@Mark Hennings Sir, does this mean that the integral is not solvable manually??? And, then, how would we even know which types of functions are integrable manually or not?? Is there a systematic algorithm, which does not involve the use of computers??

Aaghaz Mahajan - 1 year, 9 months ago

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@Aaghaz Mahajan I don't know whether or not this integral can be calculated explicitly.

I would very much doubt that any algorithm can exist that would determine when an integral can be solved manually, by which I presume you mean that its antiderivative can be explicitly expressed in terms of a standard set of functions.

And I am thoroughly happy that this should be so, since to have the whole of integration reduced to an algorithm would be desperately dull.

Mark Hennings - 1 year, 9 months ago

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