Integration Formula Explained: Li & Lj

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Discussion Overview

The discussion revolves around an integration formula involving shape functions Li and Lj, which are defined in terms of variables xi and xj. Participants seek clarification on transforming the integral into a factorial form and the underlying mathematical principles, including the potential involvement of gamma and beta functions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • A participant requests assistance in understanding an integration formula related to shape functions Li and Lj, which are defined as Li= ( xj - x ) / ( xj - xi ) and Lj= ( x - xi ) / ( xj - xi ).
  • Another participant provides a modified version of the integral, suggesting a factorial form involving the parameters α and β.
  • One participant expresses confusion about transforming the integral into the factorial form and seeks further clarification.
  • There is a request for additional context regarding the shape functions and the variables involved.
  • A reference to the book 'Applied Finite Element Analysis' by Larry Segerlind is provided as a source for the derivation of the integration formula.
  • A suggestion is made that the gamma function may be relevant to the transformation of the integral, with a recommendation to consult Abramowitz and Stegun for further information.
  • A participant acknowledges their lack of familiarity with gamma functions and requests help in evaluating specific equations related to the topic.
  • Another participant suggests obtaining a math book that covers gamma functions for better understanding.
  • A later reply indicates that the equations in question are derived from the beta function, which relates to the gamma function and factorials, and expresses gratitude for the assistance received.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the integration formula and the mathematical concepts involved. There is no consensus on the best approach to transform the integral or the specific application of gamma and beta functions, indicating multiple competing views and unresolved questions.

Contextual Notes

Some participants have not taken courses covering gamma functions, which may limit their understanding of the discussion. The derivation of the integration formula and its connection to factorials remains a point of exploration.

hash054
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Integration formula??

I am a graduate student and during my research I have come across this integration formula shows in attached image file. Can anyone help me make sense of this equation because i couldn't find any help from the literature regarding this equation.
Li and Lj are shape functions in this equations whose values Li= ( xj - x ) / ( xj - xi ) and Lj= ( x - xi ) / ( xj - xi )
 

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hash054 said:
I am a graduate student and during my research I have come across this integration formula shows in attached image file. Can anyone help me make sense of this equation because i couldn't find any help from the literature regarding this equation.
Li and Lj are shape functions in this equations whose values Li= ( xj - x ) / ( xj - xi ) and Lj= ( x - xi ) / ( xj - xi )

Here's your integral, slightly modified (using a and b as limits of integration rather than the single l (letter 'l') of your thumbnail.

$$ \int_a^b L_i^{\alpha}~L_j^{\beta}dl = \frac{\alpha ! \beta !}{(\alpha + \beta + 1)!}l$$
 
thanks for replying .. I know this is the integral.. I was asking about how can we transform it into this factorial form.. any help in that regard?
 
I put that in so people wouldn't have to open your thumbnail in another window.

Can you tell us any more about these shape functions? I'm assuming that i and j are indexes and alpha and beta are exponents. What are xi and xj?

Some context as to where this formula came up might be helpful as well.
 
This is almost certainly going to involve the gamma function. Substitute y = Lj, and (1-y)=Li. I assume that it is being integrated between y=0 and y=1. Get Abramowitz and Stegan, and look up gamma functions. The integrals in terms of y are likely to be in there.
 
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As a matter of fact, it does. You can find several copies of Abramowitz and Stegun online with Google.
 
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thanks guys for replying.. I am grateful.. yet I have not taken a course in which gamma functions were included so a little help in evaluating eq. 6.16,17 would be appreciated!
 

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hash054 said:
thanks guys for replying.. I am grateful.. yet I have not taken a course in which gamma functions were included so a little help in evaluating eq. 6.16,17 would be appreciated!
No problem. You need to get yourself a math book that covers gamma functions. Probably Kreyzig would have it; check out the table of contents on amazon. Otherwise, google gamma functions.
 
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Got it... the equation 6.16,17 are derived from beta function β (z,w).. which has a relation with gamma function and ultimately in terms of factorial.. The books you guys recommended worked for me! thanks for the help.. now i can continue! :)
 

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