Proportionality and the ln function

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Discussion Overview

The discussion revolves around the concept of proportionality, specifically when a variable x is proportional to the natural logarithm of another variable y, as seen in contexts like entropy in statistical physics. Participants explore the conditions under which x can be said to be proportional to ln(y) versus y itself, including methods for determining proportionality.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • One participant notes that x is directly proportional to y if there exists a real number λ such that x = λy.
  • Another participant suggests that if x is proportional to ln(W), then x can be expressed as x = λln(W) for some real number λ.
  • A participant questions how to determine whether x is proportional to y or ln(y), seeking rules beyond graphical methods.
  • Several participants propose calculating the ratio x/y for various values to check for a constant ratio, indicating proportionality.
  • It is mentioned that if x is directly proportional to ln(y), then the ratio x/ln(y) should also yield a constant λ.
  • Graphical interpretations are discussed, where a straight line through the origin would indicate proportionality in both cases.
  • One participant expresses a focus on ln(y) rather than y, indicating a specific interest in that relationship.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the rules for determining proportionality between x and y versus x and ln(y). Multiple approaches and methods are discussed, but no single method is universally accepted.

Contextual Notes

Some limitations include the dependence on specific definitions of proportionality and the need for further clarification on the conditions under which these relationships hold true.

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Hi,

We know that if x=2, and y=4 for example, i.e that x is directly proportional to y.
What i am wondering about is when to say that x is proportional to lny? As in the case of Entropy in Statistical Physics where S proportional to lnW?
 
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A variable [itex]x[/itex] is said to be "directly proportional" (or simply proportional) to another variable [itex]y[/itex], if one is a linear multiple of the other, i.e. there exists some real number [itex]\lambda[/itex], such that [itex]x = \lambda y[/itex].

It's termed "inversely proportional" if [itex]x = \lambda/y[/itex].

So, if [itex]x[/itex] is proportional to [itex]ln (W)[/itex], there has to exist some real number [itex]\lambda[/itex] such that [itex]x = \lambda ln(W)[/itex].
 
Last edited:
Thanks for the reply, but how to know whether it is proportional to y or lny? Is there some rule other than plotting a graph perhaps?
 
Just compute the ratio [itex]x/y[/itex] for different values of [itex]x[/itex] and their corresponding [itex]y[/itex] values. If this ratio remains a constant (the constant of proportionality [itex]\lambda[/itex] ), then [itex]x[/itex] is proportional to [itex]y[/itex].

Graphically, if [itex]x[/itex] is directly proportional to [itex]y[/itex], i.e, if [itex]x = \lambda y[/itex] then the graph of [itex]y[/itex] as a function of [itex]x[/itex] will be a straight line passing through the origin with the slope of the line equal to the constant of proportionality [itex]1/\lambda[/itex].
 
Thanks again. But still I was concentrating on the lny and not the y. Anyway, I will depend on graphical illustrations.
 
It is straightforward to generalize Ryuzaki's reply to all other functions.
Ryuzaki said:
Just compute the ratio [itex]x/y[/itex] for different values of [itex]x[/itex] and their corresponding [itex]y[/itex] values. If this ratio remains a constant (the constant of proportionality [itex]\lambda[/itex] ), then [itex]x[/itex] is proportional to [itex]y[/itex].

Graphically, if [itex]x[/itex] is directly proportional to [itex]y[/itex], i.e, if [itex]x = \lambda y[/itex] then the graph of [itex]y[/itex] as a function of [itex]x[/itex] will be a straight line passing through the origin with the slope of the line equal to the constant of proportionality [itex]1/\lambda[/itex].
For ##\ln(y)##:

Just compute the ratio [itex]x/\ln(y)[/itex] for different values of [itex]x[/itex] and their corresponding [itex]\ln(y)[/itex] values. If this ratio remains a constant (the constant of proportionality [itex]\lambda[/itex] ), then [itex]x[/itex] is proportional to [itex]\ln(y)[/itex].

Graphically, if [itex]x[/itex] is directly proportional to [itex]ln(y)[/itex], i.e, if [itex]x = \lambda \ln(y)[/itex] then the graph of [itex]\ln(y)[/itex] as a function of [itex]x[/itex] will be a straight line passing through the origin with the slope of the line equal to the constant of proportionality [itex]1/\lambda[/itex].
 
mfb said:
It is straightforward to generalize Ryuzaki's reply to all other functions.

For ##\ln(y)##:

Just compute the ratio [itex]x/\ln(y)[/itex] for different values of [itex]x[/itex] and their corresponding [itex]\ln(y)[/itex] values. If this ratio remains a constant (the constant of proportionality [itex]\lambda[/itex] ), then [itex]x[/itex] is proportional to [itex]\ln(y)[/itex].

Graphically, if [itex]x[/itex] is directly proportional to [itex]ln(y)[/itex], i.e, if [itex]x = \lambda \ln(y)[/itex] then the graph of [itex]\ln(y)[/itex] as a function of [itex]x[/itex] will be a straight line passing through the origin with the slope of the line equal to the constant of proportionality [itex]1/\lambda[/itex].

Ah, you beat me to it. :smile:
 
Thanks guys. This is close to answering my question.
 

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