Integration of a term that is squared, cubed, etc.

[hoomanya]

Hi,

I was wondering if the following statement is correct:

\int(\frac{du}{dx})^{2}dV=\int\frac{du}{dx}dV\int\frac{du}{dx}dV

Please help!

Thanks!

 

[BrianMath]

Hi,

I was wondering if the following statement is correct:

\int(\frac{du}{dx})^{2}dV=\int\frac{du}{dx}dV\int\frac{du}{dx}dV

Please help!

Thanks!

No because the left hand side has just dV, whereas the right hand side has dV^2 because you have two of them multiplying together.

 

[pmsrw3]

Hi,

I was wondering if the following statement is correct:

\int(\frac{du}{dx})^{2}dV=\int\frac{du}{dx}dV\int\frac{du}{dx}dV

Please help!

Thanks!

Counterexample: let u(x) = x, v = x.

 

[hoomanya]

Thank you both. pmsrw3 I am not bothered with solving the integral.

Can I write it as something like this then, using the divergence theorem:

\int(\frac{du}{dx})^{2}dV=\frac{1}{V}\intu.n dS\intu.n dS

Cheers!

 

[pmsrw3]

Thank you both. pmsrw3 I am not bothered with solving the integral.

?

But the example I gives shows that your speculated relationship doesn't hold. It doesn't matter whether you want an explicit solution or not.

Can I write it as something like this then, using the divergence theorem:

\int(\frac{du}{dx})^{2}dV=\frac{1}{V}\intu.n dS\intu.n dS

Cheers!

Wait a minute, I think this needs some clarification. Is dV a volume element? In how many dimensions, and what is x? What's the context?

 

[hoomanya]

Sorry, I don't get the counter example. Thank you for your quick responses.

Wait a minute, I think this needs some clarification. Is dV a volume element? In how many dimensions, and what is x? What's the context?

V is a volume integral, x is a dimension and u is displacement. The problem is 1D and the context is solid deformation and finite element analysis.

By the way, does it depend on the context or is it a mathematical relationship (if it's correct)?

Thanks again.

 

[pmsrw3]

Sorry, I don't get the counter example.

If u(x) = x, du/dx = 1. If v=x, dv=dx. The left-hand-side integral is therefore x+C1. The RHS integrals comes to (x+C2)(x+C3). Obviously the LHS and RHS aren't equal.

V is a volume integral, x is a dimension and u is displacement. The problem is 1D and the context is solid deformation and finite element analysis.

OK, then I completely don't get the question. If you're in one dimension and it's a volume integral, doesn't that mean V = x? In one dimension, the surface of a region is just two points, the left and right ends (assuming the region is contiguous), and the normal vector n is -1 at one end, +1 at the other end. In other words, the divergence theorem in one dimension is nothing but the familiar

\int_a^b \frac{du}{dx}dx = u(b) - u(a)

I think you have the idea that the fact the the integrand is the square of the derivative of something makes it special. But it doesn't, you know: ANY (well-behaved) function can be expressed as the square of the derivative of something. \int(\frac{du}{dx})^2 dV is no more special than \int u dV.

By the way, does it depend on the context or is it a mathematical relationship (if it's correct)?

Both could be true. The truth or falsehood of an equation will depend on what the symbols mean. For instance you can't tell me whether the inequality x^2 \geq 0 is true or false unless I tell you what kind of values are allowed for x.

 

[hoomanya]

Thanks a lot.If the problem was not 1D and you had to write the left hand side of the equation in terms of a product the two terms I have on the RHS(after using the divergence theorem), would there be a way? Or is there a context or a situation this or something similar would hold?

 

[pmsrw3]

Thanks a lot.If the problem was not 1D and you had to write the left hand side of the equation in terms of a product the two terms I have on the RHS(after using the divergence theorem), would there be a way? Or is there a context or a situation this or something similar would hold?

Maybe you should explain where this question comes from and what you're trying to do.

 

[hoomanya]

Thanks for trying to help. It would be too complicated to explain the entire context. I posted the question hoping this is a general mathematical relations or something close. But you helped understanding that it probably isn't. So thanks for that!

 

[hoomanya]

I think I know where the first equation I wrote is coming from... To do with finite elements ... :D