While the saya has less weight and moment than the sword, the left arm and shoulder pull back vigorously as well, especially with a “square hips” posture. In my experience, I find it has a great impact on leg stability (compared to when I don’t do it), but we perform with “square hips”. Opening the hips would also add weight in the rotation.

I feel it more when I perform from kneeling, as my left back leg can more easily rotate inward as I strike if I don’t pull back the sword correctly (it also has a lot to do with how I strike with the blade (trajectory and max speed point).

Great point about compound rotation. It’s a complex mix of center, shoulder and wrist, all with different timings…

]]>Thinking about it some more, I think that the “angular momentum trick” in the Iaidō case must work mostly on the psychological rather than physical level: a katana is far more massive than its saya, and travels further from the axis of the rotation (nominally, the spine**), and so has a much greater moment of inertia. The sayabiki can’t possibly physically counterbalance the nukitsuke, but pretending that it does may help engage the right muscle groups and produce the correct movement.

** That said, nukitsuke is also a compound rotation, with one rotation about the spine and another about the wrist once the sword has fully exited the scabbard.

]]>I have looked at it again and I think the proof in Lunardi’s book for the case k=0 extends to arbitrary k, not sure though, since it’s getting kind of late and I must just be doing stupid stuff ^^

Thank you very much nontheless!

]]>Do you have a source for the claim that the Slobodeckij-spaces are the same as the interpolation spaces?

For 0 < s 1 (I tought I had found a proof, but it turns out that it is incomplete). I know that the book by Triebel contains quite a long proof of a more general result involving Besov spaces, but I do not think I really want to get into the details of that.

The book by Luc Tartar “An Introduction to Sobolev Spaces and Interpolation Spaces” just says that there were two ways to define the fractional Sobolev spaces for s>1, either like the Slobodeckij spaces or as interpolation spaces (between L^p and W^(m,p), but that does not matter due to reiteration) and there were a few technicalities to check in order to show that these notions coincide. He then proceeds to check that the reiteration theorem is applicable, but I do not see how that gives the result already.

Maybe you can help me with that?

Thank you very much!

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