Baker–Campbell–Hausdorff (CBH) Formula question

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The forum discussion centers on the explicit Baker–Campbell–Hausdorff (CBH) formula, specifically the determination of the values of r_i and s_i for 1 ≤ i ≤ n. Users express confusion regarding how to identify these values, particularly in the context of multiple commutators. The discussion references the Wikipedia page on the CBH formula and highlights the challenge of understanding the conditions under which r_n and s_n are defined as non-negative integers. A specific example is provided where the values for a simple commutator [X,Y] are identified as r_n = s_n = r_1 = s_1 = 1.

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BlackHole213 said:
I've been trying to understand the explicit CBH formula

http://en.wikipedia.org/wiki/Baker–...er.E2.80.93Campbell.E2.80.93Hausdorff_formula

However, I don't really understand how to know what the values of r_i and s_i are, where 1\leq i\leq n.
Like it says,

Where sn and rn are non-negative integers.
The sum runs over all possible values of sn and rn, where sn + rn > 0. They apparently stand for the number of X's and Y's in the multiple commutator. (Glad I don't have to prove this! :wink:)
 
I agree, proving it would be awful, to say to least.

This may be a dumb question, but how do I know what the possible values of r_n and s_n are? I feel like I'm over-thinking this.

For example, if I consider [X,Y], then r_n=s_n=r_1=s_1=1. If I just had the BCH formula as written by Dynkin, how would I know that r_1=s_1=1 for n=1?
 

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