How to Derive the Inverse of the Sum of Two Operators?

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    Inverse Operators Sum
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Discussion Overview

The discussion revolves around the derivation of the inverse of the sum of two operators, S and P. Participants explore the mathematical formulation and validity of the expression (S+P)^{-1}=S^{-1}-S^{-1}P(S+P)^{-1}, examining whether it can be proven in a general context.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant expresses uncertainty about the possibility of deriving the inverse and seeks assistance.
  • Another participant suggests that to show the proposed inverse is valid, one must demonstrate that multiplying it by (S+P) yields the identity operator.
  • A different participant questions the correctness of the initial approach and points out a potential error in the multiplication process.
  • Further replies indicate that there are multiple errors in the calculations presented, highlighting the complexity of the derivation.
  • Several participants express curiosity about the methods used to find such inverses, discussing the validity of guessing and checking as a technique.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the validity of the proposed inverse or the correctness of the derivation steps. There are multiple competing views regarding the approach and the errors identified.

Contextual Notes

Participants note the importance of correctly applying mathematical operations and the potential for errors in the derivation process. The discussion reflects the challenges inherent in proving identities involving operators.

Who May Find This Useful

This discussion may be of interest to those studying operator theory, functional analysis, or related fields in mathematics and physics, particularly in understanding the complexities of operator inverses.

poonintoon
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Hi,
I am trying to show that for two operators S and P:
(S+P)^{-1}=S^{-1}-S^{-1}P(S+P)^{-1}
I can't get anywhere and searching on google I am not even sure if it is possible
to solve the general case but the question gives no more hints.
Any help appreciated. Thanks.
J.
 
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You'll need to show that (S^{-1}-S^{-1}P(S+P)^{-1})(S+P)=I. Id begin by multiplying both sides by S...
 
poonintoon said:
Hi,
I am trying to show that for two operators S and P:
(S+P)^{-1}=S^{-1}-S^{-1}P(S+P)^{-1}
I can't get anywhere and searching on google I am not even sure if it is possible
to solve the general case but the question gives no more hints.
Any help appreciated. Thanks.
J.
Are you sure about this? The standard way to show that A is the multiplicative inverse of B is to multiply them together to show that you get the identity. But if we multiply both sides, on the right, by S+ P we get
(S+ P)^{-1}(S+ P)= S^{-1}(S+ P)+ S^{-1}P(S+P)^{-1}(S+P)
I= S^{-1}S+ S^{-1}P+ S^{-1}= I+ S^{-1}(P+ S)
S^{-1}(P+S)= 0
which is certainly not always true!
 
Your approach was correct but you made an error, Halls.

poonintoon: Follow Hall's approach and you will have your proof.
 
Actually two errors! I lost a negative sign and a "P"!
 
Just a quick curiosity, but how would you find that inverse without having known it in the first place? Surely it wasn't found by simply trying different functions was it?
 
Anonymous217 said:
Just a quick curiosity, but how would you find that inverse without having known it in the first place?
It exists. That's all one needs to know. Well, that and the fact that operators form a ring.
 
Anonymous217 said:
Just a quick curiosity, but how would you find that inverse without having known it in the first place? Surely it wasn't found by simply trying different functions was it?
Why not? That's a well respected method!
 
HallsofIvy said:
Why not? That's a well respected method!
Well, I was hopeful of some type of actual procedure so that it could be applied to more complicated operations, but I guess not.
 
  • #10
The more you guess and check the better you get at it
 

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