Solving the Limit of sin(\pi*n/4)*\Gamma(x) Problem

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Discussion Overview

The discussion centers around evaluating the limit of the expression sin(πn/4) * Γ(x) as n approaches 0. Participants explore various approaches to this limit, including the properties of the gamma function and potential typos in the original problem statement.

Discussion Character

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

Main Points Raised

  • One participant expresses uncertainty about how to approach the limit involving the gamma function and seeks a proof, noting that Wolfram Alpha provides an answer of π/4.
  • Another participant suggests that the limit evaluates to 0, based on the substitution of n=0 into the expression.
  • A third participant questions the original formulation, proposing that it should involve Γ(n) instead of Γ(x), indicating a possible typo.
  • A later reply provides a detailed approach using the functional equation of the gamma function, breaking down the limit into components and applying L'Hôpital's rule to arrive at the limit of π/4.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the limit's value, with differing opinions on the correct interpretation of the expression and the potential typo. The discussion remains unresolved regarding the correct formulation and limit evaluation.

Contextual Notes

There are unresolved assumptions regarding the correct variable in the gamma function and the continuity of the gamma function at the point of interest. The application of L'Hôpital's rule is also contingent on the conditions being met for its use.

SumThePrimes
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I've recently been confronted with the limit as n goes to 0 of sin([itex]\pi[/itex]*n/4)*[itex]\Gamma[/itex](x) , and have no idea on how to confront the problem, as I have little familiarity with the gamma function. Is there any relatively easy ways to prove this, or at least ways that use methods not difficult to learn? I would very much like to see a proof, as wolfram alpha gives a answer of [itex]\pi[/itex]/4, and the answer is important relating to some very interesting alternating series.
edit 2: Wow, I haven't solved this problem, but if wolfram alpha is right, soon I'll be summing alternating series never summed before :) Well, ones I've never seen summed before at least...
 
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##\lim_{n \to 0} \sin(\frac{\pi n}{4}) \Gamma(x) = \sin(0)\Gamma(x) = 0##...
 
I suspect there is a typo in the original question - it should be Γ(n), not Γ(x).
 
It's actually quite easy. You know that the gamma function satisfies the functional equation [itex]\Gamma(x+1) = x \Gamma(x)[/itex], so then:

[tex]\begin{align*} \lim_{x \rightarrow 0} \sin (\pi x/4) \Gamma(x) &= \lim_{x \rightarrow 0} \frac{\sin(\pi x/4) \Gamma(x+1)}{x} \\ &= \left( lim_{x \rightarrow 0} \frac{\sin(\pi x/4)}{x} \right) \left( \lim_{x \rightarrow 0} \Gamma(x+1) \right) \\ &= lim_{x \rightarrow 0} \frac{\sin(\pi x/4)}{x}\end{align*}[/tex]

Where the last equality follows from the fact that [itex]\Gamma[/itex] is continuous and [itex]\Gamma(1) = 1[/itex]. But then by L'hopital's rule:

[tex]\lim_{x \rightarrow 0} \frac{\sin(\pi x/4)}{x} = \lim_{x \rightarrow 0} \frac{\pi}{4} \cos (\pi x/4) = \frac{\pi}{4}[/tex]
 

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