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Alexis Hazell (Sep 06 2021 at 13:10):Following the Unicode mailing list, I recently read this message:
It seems that the symbol U+22B6 has been used first by Joachim Kock from UAB Barcelona Maths Department. He is also using on a website, last modified in 2006, four years before the already mentioned paper “Polynomial functors and opetopes” had been published.[1]
The symbol has been “borrowed” from other authors, e. g. [2], [3]
Maybe Mr. Kock can provide more information about the origin of that
character and also answer the other questions.
The "U+22B6" being referred to is the symbol '⊶'.
@Joachim Kock, not sure if this query has already been passed on to you via other channels?
John Baez (Sep 06 2021 at 17:19):I think he doesn't read this very much...
Joachim Kock (Sep 07 2021 at 13:26):Thanks for the ping!
This is funny. I didn't even know ⊶ was a unicode symbol.
I started using it with André Joyal, Michael Batanin, and Jean-François Mascari in 2004 for what we called the zoom relation. (That was long before Zoom the app.) A zoom complex is a sequence of trees with circles, with a bijection between between circles in one tree and nodes of the next tree (and a bijection between nodes in one tree and leaves in the next). These bijections is the zoom relation. The symbol ⊶ is thus 'onomatopoetic'. Zoom complexes are a combinatorial model for the opetopes, and the zoom relation is implicit already in the work of Baez and Dolan.
To print the symbol in the paper, we had to use a little postscript figure in a \savebox. Nowadays it can be obtained with tikz arrow heads
-Circle and -{Circle[open]}.