I've not seen it before. The sin function needs to be applied to an angle. Is it the author's own shorthand for the angle between d and l? That would make sense in the context - as presumably it's saying when angle is zero (d || l) there is no energy cost.

I'm also thinking the same but perhaps it's more saying that the sine is a function of both these factors but the exact combination to create this scalar angle inside is unknown? Or unimportant? I'm just so confused if it's the angle between them why would they just not write the dot ext ahh

I don't think it's an accident because I've just found an actual dot product of the same vectors on another page nearby and a sine(d,l) on another one beside the dot.

Actually in India
Students are taught that (a,b) in context of vectorial quantities a and b means the angle included between them.
Is it not so in internationally accepted mathematics?

I have read the entire book front to middle so far and I'm yet to get anything. The first instance I think is page 112 of the book (low temperature Physics Christian Enss)

Is it possible that notation is explained in an appendix?
Anyway you could always try to get in touch with the author if they are still alive, or some professor who teaches this subject in your university. They would probably be able to help

I've not seen it before. The sin function needs to be applied to an angle. Is it the author's own shorthand for the angle between d and l? That would make sense in the context - as presumably it's saying when angle is zero (d || l) there is no energy cost.

I'm also thinking the same but perhaps it's more saying that the sine is a function of both these factors but the exact combination to create this scalar angle inside is unknown? Or unimportant? I'm just so confused if it's the angle between them why would they just not write the dot ext ahh

could it be angle between vectors?

It’s probably the scalar product, which is sometimes noted as (d,l), whereas the brackets of the sin is sometimes omitted

I don't think it's an accident because I've just found an actual dot product of the same vectors on another page nearby and a sine(d,l) on another one beside the dot.

Trace back to the first occurrence of this notation... It's non standard, so it should be explained in the text.

I tried this but all I can find is the first instance on page 112 with no information.

Actually in India Students are taught that (a,b) in context of vectorial quantities a and b means the angle included between them. Is it not so in internationally accepted mathematics?

Not in any textbook I've seen... But I have seen very weird local notation in the textbooks here.

Ohk Thanks for the info

I have read the entire book front to middle so far and I'm yet to get anything. The first instance I think is page 112 of the book (low temperature Physics Christian Enss)

I'd guess it's supposed to represent the inner product (dot product) of the two vectors.

Are l and d angles? If this was meant to be a product then maybe the comma was accidental and nobody noticed before publication

From the context, I’m guessskng he’s referring to the angle between the vectors.

Is it possible that notation is explained in an appendix? Anyway you could always try to get in touch with the author if they are still alive, or some professor who teaches this subject in your university. They would probably be able to help