How Many Pairs (X, Y) Meet the Condition X² + Y² ≤ N²?

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Discussion Overview

The discussion revolves around determining the number of pairs of discrete random variables (X, Y) that satisfy the condition X² + Y² ≤ N², where both X and Y range from 1 to N. The scope includes mathematical reasoning and exploratory approaches to understand the problem, particularly as N approaches infinity.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant describes their initial attempts to find patterns by using specific values of N but struggles to derive meaningful insights.
  • Another participant suggests visualizing the problem by drawing the region defined by the constraints in the X-Y plane.
  • A participant shares their experience using Mathematica to visualize the pairs that satisfy the condition, proposing that as N approaches infinity, the number of satisfying pairs approaches (πN²)/4, under the assumption that X and Y are positive.
  • There is a clarification that if the question is about counting integer pairs (X, Y) under the given condition, it may not directly relate to probability unless specific distributions for X and Y are defined.
  • A later reply emphasizes the need to calculate the probability of the event, indicating that knowing the number of pairs is essential for this calculation.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between the counting of pairs and probability, with some focusing on the mathematical counting aspect while others emphasize the probabilistic interpretation. The discussion remains unresolved regarding the best approach to derive a general formula or understanding for finite versus infinite N.

Contextual Notes

There are limitations regarding the assumptions made about the distributions of X and Y, as well as the implications of the condition for small versus large N. The discussion does not resolve how to generalize the counting for all values of N.

nezse
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Hi All,
I'm stucked computing this:
I have two discrete random variables 1≤X≤N and 1≤Y≤N. How many pairs of (X,Y) satisfy X²+Y²≤N²

I began by using a certain value for N and trying to search for patterns in the numbers that satisfy this constraint but I can't seem to get any meaningful pattern.

Any ideas?
Thanks.
 
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Try drawing it out. Draw the region 1<=X<=N and 1<=Y<=N in the X-Y plane. Then draw in the condition X^2 +Y^2 <= N^2. Does this help?
 
At first I draw it in a paper with N=9. And that give me no clue. Today, with the purpose to show you the figure, I made it again, but in Mathematica. When I see the elementes that satisfy the condition in that matrix, the answer to N→∞ become so obvius. I attach the image.

When N→∞ the amount of numbers that satisty X2+Y2≤N2 is Pi*N2/4, because X,Y>0. If X and Y be any real number equal to the restriction is an equation of a circle, treats including its inside.
For small N, I still have no idea.

Thanks, phyzguy.
 
I forgot to post the images.
 

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nezse said:
At first I draw it in a paper with N=9. And that give me no clue. Today, with the purpose to show you the figure, I made it again, but in Mathematica. When I see the elementes that satisfy the condition in that matrix, the answer to N→∞ become so obvius. I attach the image.

When N→∞ the amount of numbers that satisty X2+Y2≤N2 is Pi*N2/4, because X,Y>0. If X and Y be any real number equal to the restriction is an equation of a circle, treats including its inside.
For small N, I still have no idea.

Thanks, phyzguy.

Since the title of the thread said it was a probability question, I assumed it was in the large N limit, since this is what a probability is. Are you supposed to calculate the number of pairs as a function of N. If so, I don't know how to write a general formula.
 
If the question is simply "how many integer pairs (X,Y) are there such that X2+Y2 <= N2", then that doesn't have a lot to do with probability. Unless your random variables X and Y have some specific distribution besides the uniform distribution that you neglected to tell us, this seems to be the question you're asking.
 
I agree, my fault. The thing is i need to calculate the probability of that event, so I need to know how many pairs satisfy that condition. I already know that the probability of any pair to come up is 1/N^2, therefore I only need to know how many pairs like that can come up and multiply that number by 1/N^2. That's why I asked for the number of pairs.
 

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