Solving for a factor in a large sum

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Discussion Overview

The discussion revolves around solving for a factor, A0, within a sum represented by the formula T = Æ© It * A0t, where the sum runs from t=1 to N. Participants explore methods for finding A0, addressing the complexity of the calculations involved.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • Dan presents a formula involving a sum and expresses difficulty in solving for A0 due to messy calculations.
  • One participant asks for clarification on the variable It, which Dan explains as a series of observations with no well-defined relation to t.
  • Another participant suggests that the equation resembles a general polynomial of order N and recommends using numerical methods, specifically Newton's method, to find roots of the function.
  • Dan clarifies that N is not large but varies from case to case, which contributes to the complexity of the sum.
  • There is a clarification that A0 has t as an exponent, not as a subscript.
  • A participant notes that for small values of N (like 3 or 4), analytic methods could still be applicable, but numerical techniques like Newton's method are generally easier to implement.
  • Discussion includes a reference to the quartic formula for N=4, highlighting the complexity of solving higher-order polynomials.

Areas of Agreement / Disagreement

Participants generally agree on the complexity of the problem and the potential usefulness of numerical methods, but there is no consensus on the best approach, particularly regarding the applicability of analytic methods for small values of N.

Contextual Notes

The discussion does not resolve the assumptions regarding the nature of the observations It or the implications of varying N on the solution process. The complexity of the polynomial and the choice between numerical and analytic methods remain open questions.

IsomaDan
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Dear everyone.

First of all Merry Xmas, when everybody gets to that.

I have a problem solving for a factor within a sum.

My formula looks as follows:

T = Æ© It * A0t

The sum runs from t=1 to N, and the aim is to solve for A0, but all my calculations end up extremely messy.

All the best,

Dan
 
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What is It?
 
Thanks for the reponse.

That is the t'th observation of I. They have no well-defined relation to t. In other words; just a bunch of numbers.
 
In that case, your equation seems to be a general polynomial of order N. I guess N is probably fairly large (as in... not 2 or 3). You will need to use some numerical method to find roots of the function f(A0) = Æ© It * A0t - T.

I suggest Newton's method, it should be convenient to implement, because you can easily compute the derivative of the function analytically.
 
Actually it is not that large (N, that is), it is just that it varies a lot from case to case and hence I have written it as a sum.

Thanks so much for the response. I will try to see if it gets me any further!
 
And just to be clear. The A_0 has t as their exponent. It is not a subscript!
 
If N is equal to 3 or 4, the polynomial is still solvable by analytic methods, but in general it is easier to use a numerical technique like Newton's Method (which is very easy to program if you choose to do that).

The N = 3 case isn't this bad, but consider the solutions for N = 4:

http://upload.wikimedia.org/wikipedia/commons/9/99/Quartic_Formula.svg

Of course, if you know ##a## is a solution to the polynomial, you can long divide by ##(x-a)## to reduce the degree by 1.
 

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