- #1
fab13
- 312
- 6
I am using from the following Euler equations :
$$\dfrac{\partial f}{\partial u^{i}}-\dfrac{\text{d}}{\text{d}s}\bigg(\dfrac{\partial f}{\partial u'^{i}}\bigg) =0$$
with function ##f## is equal to :
$$f=g_{ij}\dfrac{\text{d}u^{i}}{\text{d}s}\dfrac{\text{d}u^{j}}{\text{d}s}$$
and we have $$f=g_{ij} u'^{i}u'^{j}=1$$ (with ##u'^{i}=\dfrac{\text{d}u^{i}}{\text{d}s}##)
My issue is this :
starting from the expression of function ##f## and Euler equations, I would like to get :
$$\dfrac{\text{d}}{\text{d}s}(g_{ij}u'^{j})-\dfrac{1}{2}\partial_{i}g_{jk}u'^{j}u'^{k}=g_{ij}\dfrac{\text{d}u'^{j}}{\text{d}s}+(\partial_{k}g_{ij}-\dfrac{1}{2}\partial_{i}g_{jk})u'^{j}u'^{k}=0$$
but I can't obtain it.
Finally, I should get the general form of geodesics equation, i.e :
$$g_{ij}\dfrac{\text{d}u'^{j}}{\text{d}s}+\Gamma_{ijk}u'^{j}u'^{k}$$
For the moment, concerning this problem (which is about factor 1/2), IF I SUBTRACT INTENTIONALLY THE TERM ##\dfrac{1}{2}\partial_{i}g_{jk}u'^{j}u'^{k}## to the expression ##\dfrac{\text{d}}{\text{d}s}(g_{ij}u'^{j})##, so I can write :
$$\dfrac{\text{d}}{\text{d}s}(g_{ij}u'^{j})-\dfrac{1}{2}\partial_{i}g_{jk}u'^{j}u'^{k}=$$
$$\dfrac{\partial g_{ij}}{\partial u^{k}}u'^{j}\dfrac{\text{d}u^{k}}{\text{d}s}+g_{ij}\dfrac{\text{d}u'^{j}}{\text{d}s}-\dfrac{1}{2}\partial_{i}g_{jk}u'^{j}u'^{k}=$$
$$\partial_{k} g_{ij} u'^{k}u'^{j}+g_{ij}\dfrac{\text{d}u'^{j}}{\text{d}s}-\dfrac{1}{2}\partial_{i}g_{jk}u'^{j}u'^{k}=$$
$$g_{ij}\dfrac{\text{d}u'^{j}}{\text{d}s}+(\partial_{k}g_{ij}-\dfrac{1}{2}\partial_{i}g_{jk})u'^{j}u'^{k}=0$$
from which I deduce :
$$g_{ij}\dfrac{\text{d}u'^{j}}{\text{d}s}+\Gamma_{ijk}u'^{j}u'^{k}$$
BUT I HAVE SUBTRACTED INTENTIONALLY THE TERM ##\dfrac{1}{2}\partial_{i}g_{jk}u'^{j}u'^{k}## ,
Now I would like to know how this term( ##\dfrac{1}{2}\partial_{i}g_{jk}u'^{j}u'^{k}## ) can be obtained from Euler equations , especially the factor ##\dfrac{1}{2}##), I think it comes from the following term of Euler equations :
$$\dfrac{\partial f}{\partial u^{i}}$$
From my point of view, $$\dfrac{\partial f}{\partial u^{i}}=\partial_{i}g_{jk}u'^{j}u'^{k}$$
and not $$\dfrac{1}{2}\partial_{i}g_{jk}u'^{j}u'^{k}$$.
but I don't know to make appear ##\dfrac{1}{2}## factor.
If someone could help me to solve this little issue, Regards
$$\dfrac{\partial f}{\partial u^{i}}-\dfrac{\text{d}}{\text{d}s}\bigg(\dfrac{\partial f}{\partial u'^{i}}\bigg) =0$$
with function ##f## is equal to :
$$f=g_{ij}\dfrac{\text{d}u^{i}}{\text{d}s}\dfrac{\text{d}u^{j}}{\text{d}s}$$
and we have $$f=g_{ij} u'^{i}u'^{j}=1$$ (with ##u'^{i}=\dfrac{\text{d}u^{i}}{\text{d}s}##)
My issue is this :
starting from the expression of function ##f## and Euler equations, I would like to get :
$$\dfrac{\text{d}}{\text{d}s}(g_{ij}u'^{j})-\dfrac{1}{2}\partial_{i}g_{jk}u'^{j}u'^{k}=g_{ij}\dfrac{\text{d}u'^{j}}{\text{d}s}+(\partial_{k}g_{ij}-\dfrac{1}{2}\partial_{i}g_{jk})u'^{j}u'^{k}=0$$
but I can't obtain it.
Finally, I should get the general form of geodesics equation, i.e :
$$g_{ij}\dfrac{\text{d}u'^{j}}{\text{d}s}+\Gamma_{ijk}u'^{j}u'^{k}$$
For the moment, concerning this problem (which is about factor 1/2), IF I SUBTRACT INTENTIONALLY THE TERM ##\dfrac{1}{2}\partial_{i}g_{jk}u'^{j}u'^{k}## to the expression ##\dfrac{\text{d}}{\text{d}s}(g_{ij}u'^{j})##, so I can write :
$$\dfrac{\text{d}}{\text{d}s}(g_{ij}u'^{j})-\dfrac{1}{2}\partial_{i}g_{jk}u'^{j}u'^{k}=$$
$$\dfrac{\partial g_{ij}}{\partial u^{k}}u'^{j}\dfrac{\text{d}u^{k}}{\text{d}s}+g_{ij}\dfrac{\text{d}u'^{j}}{\text{d}s}-\dfrac{1}{2}\partial_{i}g_{jk}u'^{j}u'^{k}=$$
$$\partial_{k} g_{ij} u'^{k}u'^{j}+g_{ij}\dfrac{\text{d}u'^{j}}{\text{d}s}-\dfrac{1}{2}\partial_{i}g_{jk}u'^{j}u'^{k}=$$
$$g_{ij}\dfrac{\text{d}u'^{j}}{\text{d}s}+(\partial_{k}g_{ij}-\dfrac{1}{2}\partial_{i}g_{jk})u'^{j}u'^{k}=0$$
from which I deduce :
$$g_{ij}\dfrac{\text{d}u'^{j}}{\text{d}s}+\Gamma_{ijk}u'^{j}u'^{k}$$
BUT I HAVE SUBTRACTED INTENTIONALLY THE TERM ##\dfrac{1}{2}\partial_{i}g_{jk}u'^{j}u'^{k}## ,
Now I would like to know how this term( ##\dfrac{1}{2}\partial_{i}g_{jk}u'^{j}u'^{k}## ) can be obtained from Euler equations , especially the factor ##\dfrac{1}{2}##), I think it comes from the following term of Euler equations :
$$\dfrac{\partial f}{\partial u^{i}}$$
From my point of view, $$\dfrac{\partial f}{\partial u^{i}}=\partial_{i}g_{jk}u'^{j}u'^{k}$$
and not $$\dfrac{1}{2}\partial_{i}g_{jk}u'^{j}u'^{k}$$.
but I don't know to make appear ##\dfrac{1}{2}## factor.
If someone could help me to solve this little issue, Regards