Differential forms, help finding the exterior dervative in dimensions greater than 3by saminator910 Tags: exterior derivative 

#1
Jan613, 07:32 PM

P: 81

So say I have a n1 form
[itex]\sum^{n}_{i=1}x^{2}_{i}dx_{1}...\widehat{dx_{i}}...dx_{n}[/itex] and I want to find the exterior derivative, how do I know where to put which partial derivative for each term, would it simply be?? [itex]\sum^{n}_{i=1} \frac{∂x^{2}_{i}}{∂x_{i}}dx_{i}dx_{1}...\widehat{dx_{i}}...dx_{n}[/itex] hopefully this will clarify, for this 2form [itex]\alpha=x_{1}x_{2}dx_{2}dx_{4} + x_{3}x_{4}dx_{1}dx_{2}[/itex] how would one go about finding the exterior derivative? I have no idea which partials to put where, this is simple for a normal ℝ^{3} 2 form, but I have no idea here. let me know if I need to clarify. 



#2
Jan713, 06:37 AM

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hi saminator910!
i'll rewrite your question slightly, since i find it a little confusing … how do i find the exterior derivative of an n1 form [itex]\sum^{n}_{i=1}f_i(x_1,\cdots x_n)dx_{1}..\cdots\widehat{dx_{i}}\cdots dx_{n}[/itex] ?i find it easier to write it as [itex](\sum^{n}_{l=1}d_j)\wedge\sum^{n}_{i=1}f_i(x_1,...x_n)dx_{1}\wedge\cdot s \widehat{dx_{i}}\cdots\wedge dx_{n}[/itex] then everything except j = i is zero, and you get [itex](\sum^{n}_{i=1}\partial f_i(x_1,...x_n)/\partial x_i)\ \ dx_{1}\wedge\cdots \wedge dx_{n}[/itex] 



#3
Jan713, 06:37 PM

P: 81

thanks a lot, this seems to make sense. That is actually in ℝ^{4}, how would one solve that? I was saying it would be simple for a 2form in ℝ^{3}, but it's difficult in ℝ^{4}.




#4
Jan813, 03:54 AM

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Differential forms, help finding the exterior dervative in dimensions greater than 3
ok, then eg [itex]d\wedge (x_1x_2\,dx_2\wedge dx_4)[/itex]
= [itex]\partial (x_1x_2)/\partial x_1\ (dx_1\wedge dx_2\wedge dx_4)[/itex] [itex]+ \partial (x_1x_2)/\partial x_3\ (dx_3\wedge dx_2\wedge dx_4)[/itex] = [itex]x_2\ (dx_1\wedge dx_2\wedge dx_4)[/itex] 



#5
Jan813, 08:37 PM

P: 81

Okay thanks alot , that really makes things clearer. So for the 2 form in [itex]ℝ^{4}[/itex]
I'm going through this step by step, just in case I make a mistake... [itex]β=x_{1}x_{2}dx_{3}dx_{4}+x_{3}x_{4}dx_{1}dx_{2}[/itex] [itex]dβ=d(x_{1}x_{2})dx_{3}dx_{4}+d(x_{3}x_{4})dx_{1}dx_{2}[/itex] [itex]dβ=(x_{2}dx_{1}+x_{1}dx_{2})dx_{3}dx_{4}+(x_{4}dx_{3}+x_{3}dx_{4})dx_{1 }dx_{2}[/itex] now from here is where I think I'm doing something wrong, I "distribute" if you will, the dx's outside the parenthesis and get [itex]dβ=x_{2}dx_{1}dx_{3}dx_{4}+x_{1}dx_{2}dx_{3}dx_{4}+x_{4}dx_{3}dx_{1}dx_ {2}+x_{3}dx_{4}dx_{1}dx_{2}[/itex] but the supposed answer is this, notice the switched dx's in the last two terms, why do I need to do this? [itex]dβ=x_{2}dx_{1}dx_{3}dx_{4}+x_{1}dx_{2}dx_{3}dx_{4}+x_{4}dx_{1}dx_{2}dx_ {3}+x_{3}dx_{1}dx_{2}dx_{4}[/itex] 



#6
Jan913, 04:35 AM

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hi saminator910!
##dx_{3}\wedge dx_{1}\wedge dx_{2}## is the same as ##dx_{1}\wedge dx_{2}\wedge dx_{3}## (it's an even number of exchanges, so there's no minusone factor) but the latter looks better 



#7
Jan913, 06:14 PM

P: 81

Thanks alot!




#8
Jan1013, 08:53 AM

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[itex]= (x_2dx_1+ x_1dx_2)(dx_2dx_4)+ (x_4dx_3+ x_3dx_4)(dx_1dx_2)[/itex] [itex]= (x_2dx_1dx_2dx_4+ x_1dx_2dx_2dx_4)+ (x_4dx_3dx_1dx_2+ x_3dx_4dx_1dx_2)[/itex] Now use the fact that the multiplication is "antisymmetric" (which immediately implies that [itex]dx_2dx_2= 0[/itex]) to write that as [itex]x_2dx_1dx_2dx_4+ x_4dx_1dx_2dx3+ x_3dx_1d_2d_3[/itex] The first term, [itex]x_2dx_1dx_2dx_4[/itex] already has the differentials in the "correct" order. The last two, [itex]x_4dx_3dx_1dx_2[/itex] and [itex]x_3dx_4dx_1dx_2[/itex], each require two transpositions, [itex]x_4dx_3dx_1dx_2[/itex] to [itex]x_4d_1dx_3dx_2[/itex] to [itex](x_4dx_1dx_2dx_3)[/itex] and [itex]x_3dx_4dx_1dx_2[/itex] to [itex]x_3dx_1dx_4dx_2[/itex] to [itex](x_3dx_1dx_2dx_4)[/itex] and so have no net change in sign. (What is the "correct" order is, of course, purely conventional.) 


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