So a bit of an off-topic question, but I think a lot of you here have an interest in math. If you had an expression you wanted to put in index notation given for one value of i as:
Where x is a vector with i = 1 → 3:
Where superscripts a and b refer to particles, for example see this picture:
https://www.researchgate.net/publication/270278277_A_Lagrangian-Lagrangian_framework_for_the_simulation_of_rigid_and_deformable_bodies_in_fluid_Multibody_Dynamics_Computational_Methods_and_Applications/figures?lo=1&utm_source=google&utm_medium=organic - Picture source no need to click.
How would you write this in index notation using Einstein summation convention? I’ve been trying something like this:
Where the nominator seems fine to me, but the denominator I just don’t see how to make it “sum” as I want.
It is primarily the subscripts I want help with, the superscripts are fine to me.