How to Derive d^* Without Using the Hodge Star Operator?

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Discussion Overview

The discussion revolves around deriving the adjoint operator \(d^*\) of the exterior derivative \(d\) on a Riemannian manifold without utilizing the Hodge star operator. Participants explore the implications of using the inner product induced by the Riemannian metric instead.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that the adjoint \(d^*\) is typically derived using the Hodge star operator and asks for an alternative method using the inner product directly.
  • Another participant suggests that any good differential geometry text should cover this topic and mentions specific books that might be useful.
  • A participant expresses interest in differential forms and their applications to physics but feels unprepared to engage deeply in the discussion.
  • One participant questions the necessity of avoiding the Hodge star operator, proposing that it might be acceptable to replace it with a formula that calculates its effects instead.
  • A later reply clarifies that the original inquiry is about finding a different method to obtain adjoints without relying on the Hodge star, indicating a desire for a more fundamental approach.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best approach to derive \(d^*\) without the Hodge star operator. Multiple viewpoints and methods are presented, indicating ongoing debate and exploration of the topic.

Contextual Notes

Some participants express uncertainty about the texts mentioned and their relevance to the discussion, while others highlight their own limitations in engaging with the technical details of the topic.

HenryGomes
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Usually the adjoint to the exterior derivative d^* on a Riemannian manifold is derived using the inner product
\langle\langle\lambda_1,\lambda_2\rangle\rangle:=\int_M\langle\lambda_1,\lambda_2\rangle\mbox{vol}=\int_M\lambda_1\wedge*\lambda_2
where \lambda are p-forms and * is the Hodge duality operator taking p-forms to (n-p)-forms which is defined by the above equation where \langle\cdot,\cdot\rangle is the canonical inner product induced on p-forms by the Riemannian metric g (it is just the tensor p-product of the inverse metric).
It is quite easy to derive d^*=*d*. But does anyone know how to do this without using Hodge star operator, through the g-induced inner product directly?
 
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what text are you referencing?
 
Any good differential geometry book should have this. One I like, which is for physicist's is John Baes' "Knots, Gauge Theory and Gravity". You might also want to check out Bleecker's " Variational Principles in Gauge Theories". I re-read the post and it seemed a bit badly written. So just to spell out what I meant:
The Hodge star is defined by:
\langle \lambda_1,\lambda_2\rangle vol=\lambda_1\wedge*\lambda_2
Find the adjoint of d without using Hodge star, just the canonically induced metric \langle\cdot,\cdot\rangle
 
thanks, Henry.

I have R.W.R. Darling's "Differential Forms and Connections". Formal. Not written for physicists. And a 1985 dover reprint, Dominic G. B. Edelen, "Applied Exterior Calculus". Neither one have I read yet.

Perhaps you could tell me if either of these texts might be worth trying.

I don't want to be misleading. I find differential forms fascinating in their application to physics. Sadly, I'm not capable of responding to your past three posts, as yet, but I would surely like to get to that point. From what I've seen over the past 6 weeks, Hurkyl seems to talk of differential forms with some authority. You might try buttonholing him for some input.

-deCraig
 
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Silly question -- if you want to express * d * without the Hodge stars, what's wrong with simply replacing them with a formula that calculates them?


Or... maybe your question is more fundamental? You call it the adjoint, so I assume

\langle d^* f, g \rangle = \langle f, dg \rangle?

Was that what you wanted? Or maybe something like d^*f is the transpose of the tangent multi-vector \langle f, d \_\_\_ \rangle?
 
Last edited:
Because it is not really the answer I am looking for, but to apply a different method to obtain adjoints when we do not have the Hodge star. Thanks for the answer!
 

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