Double Integral of min(x,y): How Do I Solve This Homework Problem?

In summary, the conversation discusses how to solve a double integral with both limits from 0 to 1 using a signum function or by splitting the integration in the unit square along the line x=y. One person also made a mistake in their initial solution but was able to correct it with the help of the other person.
  • #1
Roni1985
201
0

Homework Statement




[tex]\int[/tex][tex]\int[/tex]min(x,y)*dx*dy
both limits are from 0 to 1

what is the general solution of this question ?

Homework Equations





The Attempt at a Solution


I attempted to solve by saying that x<y, finding the integral
and then, y<x and finding its integral.
meaning, I divided my double integral into two double integrals that go from 0 to 1.

Actually, I have never done this before and this was just a guess.

Any help would be appreciated.
 
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  • #2
Roni1985 said:

Homework Statement




[tex]\int[/tex][tex]\int[/tex]min(x,y)*dx*dy
both limits are from 0 to 1

what is the general solution of this question ?

Homework Equations





The Attempt at a Solution


I attempted to solve by saying that x<y, finding the integral
and then, y<x and finding its integral.
meaning, I divided my double integral into two double integrals that go from 0 to 1.

Actually, I have never done this before and this was just a guess.

Any help would be appreciated.

That's a really good guess. I can't think of a better way to handle it.
 
  • #3
dickdick said:
That's a really good guess. I can't think of a better way to handle it.

Hello,

Well, I think the answer is wrong because I need to get is 1/3 if I remember correctly... but I keep getting 1.


Thanks.
 
  • #4
Roni1985 said:
Hello,

Well, I think the answer is wrong because I need to get is 1/3 if I remember correctly... but I keep getting 1.


Thanks.

Sure. Split it into two integrals. I'd be curious to see how you manage to get 1. Can you show your work? 1/3 is correct.
 
  • #5
Roni1985 said:

Homework Statement




[tex]\int[/tex][tex]\int[/tex]min(x,y)*dx*dy
both limits are from 0 to 1

what is the general solution of this question ?

Homework Equations





The Attempt at a Solution


I attempted to solve by saying that x<y, finding the integral
and then, y<x and finding its integral.
meaning, I divided my double integral into two double integrals that go from 0 to 1.

Actually, I have never done this before and this was just a guess.

Any help would be appreciated.

Have you tried using a signum function?

You can use a signum function to calculate either a minimum or a maximum. However I'm not sure if an integral exists for it. If one did I would think that it would be some special function that's non-polynomial.

In case your interested here is how you calculate min(x,y)

min(x,y) = y * (signum(x-y)+1)/2 + x * (signum(y - x)+1)/2. The signum function in this case is defined as:

signum(x) = -1 if x < 0, +1 if x > 0, 0 if x = 0.

This kind of function has a Fourier series so you may want to ask someone who knows about these kind of functions.
 
  • #6
chiro said:
Have you tried using a signum function?

You can use a signum function to calculate either a minimum or a maximum. However I'm not sure if an integral exists for it. If one did I would think that it would be some special function that's non-polynomial.

In case your interested here is how you calculate min(x,y)

min(x,y) = y * (signum(x-y)+1)/2 + x * (signum(y - x)+1)/2. The signum function in this case is defined as:

signum(x) = -1 if x < 0, +1 if x > 0, 0 if x = 0.

This kind of function has a Fourier series so you may want to ask someone who knows about these kind of functions.

It is seriously not that complicated. Just split the integration in the unit square along the line x=y.
 
  • #7
dickdick said:
Sure. Split it into two integrals. I'd be curious to see how you manage to get 1. Can you show your work? 1/3 is correct.

OMG, it is 1/3... I didn't change the limits of the integrals even after I divided them...


Thanks for your help...
 

What is a double integral?

A double integral is a mathematical concept used in calculus to calculate the volume under a surface in a two-dimensional space. It is represented by a curved "S" symbol and consists of two integrals, an outer integral and an inner integral.

What is the "min" function in a double integral?

The "min" function in a double integral refers to the minimum value between two variables. In the context of min(x,y), it means that the value of the function at a certain point is equal to the smaller of the two variables x and y.

Why is the "min" function used in a double integral?

The "min" function is used in a double integral to determine the range of integration for the inner integral. Since the value of the function is equal to the smaller of the two variables, it helps to set the boundaries for the integration in order to accurately calculate the volume under the surface.

How do you solve a double integral of min(x,y)?

To solve a double integral of min(x,y), you must first set up the integral by determining the range of integration for both the inner and outer integrals. Then, you can evaluate the integral by using the standard techniques of integration, such as substitution or integration by parts.

What are the applications of a double integral of min(x,y)?

A double integral of min(x,y) has various applications in real-world problems, such as calculating the volume of a solid with varying cross-sections, finding the center of mass of an object, and determining the probability of a certain event in statistics.

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