- #1
Diffeomorphic
- 23
- 0
Hi there,
I'm writing a research paper and have hit a roadblock, (wikipedia did not help) and one of my collaborators sent me an e-mail that I do not understand.
I am attempting to find when the following functional is stationary:
[tex]
T = \int\limits_{\lambda_{1}}^{\lambda_{2}}
\sqrt{\sum_{I=1}^{n} \sum_{i=1}^{d}
\left(\frac{d}{d\lambda}\left(\sum_{j=1}^{d} s(\lambda)R_{j}^{i}(\lambda)(q_{I}^{j}( \lambda) + a^{j}(\lambda))\right)\right)^2} d\lambda
[/tex]
This is not necessarily important for anyone who can help answer this question to understand but it helps gives context:
T is the length of all possible curves in configuration space between two particular systems of particles. I represents the number of particles, while i and j are two indices defining the dimension. s is a function describing scale, R is a function describing rotation, q describes the system of particles, and a describes translation. So, the similarity group has 3 elements: the subgroups T^{d} and SO(d) plus the element k, defined by a, R, and s, respectively.
Now, math wise I'm attempting to use the euler-lagrange equation to solve for constraints on s, R, q, and a. Lambda is just a parameter defining where on the trial curve the system of particles is. So, given the euler-lagrange:
[tex]
\frac{\partial{f}}{\partial{x}} - \frac{d}{d \lambda}
\left(\frac{\partial{f}}{\partial{\dot{x}}}\right) = 0
[/tex]
Where f is the integrand of T and x is the variable defining g(\lambda) such that if g(\lambda) is taken to be s(\lambda) then x would be the scaling constant k. In order to avoid a long and drawn out derivation of the conditions on T, the following substitution must be observed, where [tex]\dot{g}(\lambda) = \frac{d}{d \lambda}(g(\lambda))[/tex]:
[tex]
\varepsilon_{I}^{i} = \frac{d}{d \lambda} \left( \sum\limits_{j=1}^{d} s(\lambda) R_{j}^{i}(\lambda)(q_{I}^{j}(\lambda) + a^{j}(\lambda)) \right) [/tex]
Now, I essentially just need the rules for using partial differentiation with respect to Einstein notation.
I got to the point where using regular derivative rules I need to compute:
[tex]
\frac{\partial}{\partial{a^{j}}} \left(\sum\limits_{j} (\dot{s} R_{j}^{i} q_{I}^{j} + s \dot{R}_{j}^{i} q_{I}^{j} + s {R}_{j}^{i} \dot{q}_{I}^{j} + \dot{s} R_{j}^{i} a^{j} + s \dot{R}_{j}^{i} a^{j} + s R_{j}^{i} \dot{a}^{j}) \right)
[/tex]
Now, I am unsure what to do. My collaborator said this:
-
"You are summing over $j$ in the bracketed expression but, in (6), you want to take derivatives with respect to $a_j$. These two $j$ are not the same! Call one of them $k$ (say the one that is summed over). Then you get,
[tex]
\frac {\partial a_j} {\partial a_k} = \delta^j_k
[/tex]
where [tex]\delta^j_k[/tex] is 1 if $j=k$ and zero otherwise. This will give a vector with index $j$ when you perform the sum over $k$, which will now be trivial. You must have a vector with these components because you are differentiating with respect to a vector.
In the case of [tex]q_I^i[/tex], you will get two delta functions [tex]\delta^I_K[/tex] and [tex]\delta^i_k[/tex]. The sums over $I$ and $k$ can then be trivially taken. You will see that this leads to a very different result."
-
Can anyone help me interpret this or help me out? If you have time I would love to just send you the paper on what I've done so far, it's about 17 pages and 4 of them are incorrect because they lead off of a mistake I made right at this point. If you need any clarification whatsoever I can e-mail you the paper in a .pdf or .tex if you post your e-mail here or send it to me in a pm.
If anyone can help me that would be so unbelievably helpful, I've asked a professor at my university and he was not familiar with Einstein notation so he could not help.
Thanks,
-Sam Reid
I'm writing a research paper and have hit a roadblock, (wikipedia did not help) and one of my collaborators sent me an e-mail that I do not understand.
I am attempting to find when the following functional is stationary:
[tex]
T = \int\limits_{\lambda_{1}}^{\lambda_{2}}
\sqrt{\sum_{I=1}^{n} \sum_{i=1}^{d}
\left(\frac{d}{d\lambda}\left(\sum_{j=1}^{d} s(\lambda)R_{j}^{i}(\lambda)(q_{I}^{j}( \lambda) + a^{j}(\lambda))\right)\right)^2} d\lambda
[/tex]
This is not necessarily important for anyone who can help answer this question to understand but it helps gives context:
T is the length of all possible curves in configuration space between two particular systems of particles. I represents the number of particles, while i and j are two indices defining the dimension. s is a function describing scale, R is a function describing rotation, q describes the system of particles, and a describes translation. So, the similarity group has 3 elements: the subgroups T^{d} and SO(d) plus the element k, defined by a, R, and s, respectively.
Now, math wise I'm attempting to use the euler-lagrange equation to solve for constraints on s, R, q, and a. Lambda is just a parameter defining where on the trial curve the system of particles is. So, given the euler-lagrange:
[tex]
\frac{\partial{f}}{\partial{x}} - \frac{d}{d \lambda}
\left(\frac{\partial{f}}{\partial{\dot{x}}}\right) = 0
[/tex]
Where f is the integrand of T and x is the variable defining g(\lambda) such that if g(\lambda) is taken to be s(\lambda) then x would be the scaling constant k. In order to avoid a long and drawn out derivation of the conditions on T, the following substitution must be observed, where [tex]\dot{g}(\lambda) = \frac{d}{d \lambda}(g(\lambda))[/tex]:
[tex]
\varepsilon_{I}^{i} = \frac{d}{d \lambda} \left( \sum\limits_{j=1}^{d} s(\lambda) R_{j}^{i}(\lambda)(q_{I}^{j}(\lambda) + a^{j}(\lambda)) \right) [/tex]
Now, I essentially just need the rules for using partial differentiation with respect to Einstein notation.
I got to the point where using regular derivative rules I need to compute:
[tex]
\frac{\partial}{\partial{a^{j}}} \left(\sum\limits_{j} (\dot{s} R_{j}^{i} q_{I}^{j} + s \dot{R}_{j}^{i} q_{I}^{j} + s {R}_{j}^{i} \dot{q}_{I}^{j} + \dot{s} R_{j}^{i} a^{j} + s \dot{R}_{j}^{i} a^{j} + s R_{j}^{i} \dot{a}^{j}) \right)
[/tex]
Now, I am unsure what to do. My collaborator said this:
-
"You are summing over $j$ in the bracketed expression but, in (6), you want to take derivatives with respect to $a_j$. These two $j$ are not the same! Call one of them $k$ (say the one that is summed over). Then you get,
[tex]
\frac {\partial a_j} {\partial a_k} = \delta^j_k
[/tex]
where [tex]\delta^j_k[/tex] is 1 if $j=k$ and zero otherwise. This will give a vector with index $j$ when you perform the sum over $k$, which will now be trivial. You must have a vector with these components because you are differentiating with respect to a vector.
In the case of [tex]q_I^i[/tex], you will get two delta functions [tex]\delta^I_K[/tex] and [tex]\delta^i_k[/tex]. The sums over $I$ and $k$ can then be trivially taken. You will see that this leads to a very different result."
-
Can anyone help me interpret this or help me out? If you have time I would love to just send you the paper on what I've done so far, it's about 17 pages and 4 of them are incorrect because they lead off of a mistake I made right at this point. If you need any clarification whatsoever I can e-mail you the paper in a .pdf or .tex if you post your e-mail here or send it to me in a pm.
If anyone can help me that would be so unbelievably helpful, I've asked a professor at my university and he was not familiar with Einstein notation so he could not help.
Thanks,
-Sam Reid