Trying to find integral using laplace method

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Discussion Overview

The discussion revolves around evaluating the integral ∫∞x exp(N(lns-s)) ds using the Laplace method, specifically addressing the cases when x is greater than, less than, and equal to 1. Participants explore the implications of the stationary point's position relative to the integration limits.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested, Mathematical reasoning

Main Points Raised

  • One participant seeks to understand how to integrate the given expression for different ranges of x, noting that the maximum point occurs at x=1.
  • Another participant points out that x is treated as a constant in the integral, questioning whether there was a typographical error regarding the variables.
  • A clarification is made that the integral is from x to infinity, with the function being exp(N(lns-s)) integrated with respect to s.
  • One participant suggests that the expression can be simplified to a form resembling the Gamma function, proposing a transformation to facilitate the integration.
  • Another participant confirms that N is a large integer but expresses confusion about calculating the integral specifically when x>1.
  • A later reply introduces the concept of the incomplete gamma function as a potential avenue for addressing the integral.

Areas of Agreement / Disagreement

Participants express varying degrees of understanding and approaches to the problem, with no consensus reached on how to handle the integral for x>1. Multiple competing views and methods are presented.

Contextual Notes

There are unresolved assumptions regarding the value of N and the implications of the stationary point's location on the integration process. The discussion also highlights potential transformations that may or may not lead to a solution.

Mathsone
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∫∞x exp(N(lns-s)) ds

how do integrate this when x>1 x<1 and x=1 using laplace method?
the maximum point is at x=1


i have the answer for x<1 and x=1

but I am struggling for x>1 as the stationary point is no longer inside the interval.
 
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Hey Mathsone and welcome to the forums.

In your integral x is not being integrated with respect to which means it is a constant which doesn't have any effect on the integral.

Is the 'x' meant to be an 's' or the 's' meant to be an 'x'? (Both end up being the same thing).
 
the integral is from x to infinity, the function is exp(N(lns-s)) which is being integrated with respect to s.

i think the format of my integral just got meddled up when i was typing out the question. sorry bout that.
 
Do you have a particular value for N or do you have to show that it can or can't be solved for some appropriate N (like integers).

For a start your expression simplifies down to s^N x e^(-sN) which is nearly in the form of a Gamma function. I have transformed your equation into a gamma function with a simple transformation using the substituion t = nS and then changing the limits to get it into a Gamma function.

Using this information, does this help you with your question?
 
N is a large integer. hm...well yes, but i did do that. its jus that i don't understand how to calculate the integral when x>1
 
Mathsone said:
N is a large integer. hm...well yes, but i did do that. its jus that i don't understand how to calculate the integral when x>1

Are you aware of the incomplete gamma function?
 

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