Here are some facts, and my speculations.
Japanese as Text
In non-technical context, we can use arbitrary text (just like in English) in equations:
長方形について， "面積 = 縦 × 横"
is a natural way of expressing the idea (so is "area = length × width").
Elaborating on this,
仕事 = ∫ 力・d(位置)
is very rarely seen but would be acceptable by most readers.
Japanese character as a symbol
Interestingly, I found a rare case where a Japanese character is used in the way you are looking for. On “The stack of higher internal categories and stacks of iterated spans”, page 13, they have this notation: "The Yoneda embedding よ : C → P(C)" (No
\mathcal shown here)
The author seems to have decided that 「よ」 for 「米田」 is a good choice.
Latin and Greek alphabets are used everywhere. ℵ from Hebrew is a conventional notation in set theory, and ב is used in similar contexts. I believe what alphabets appear in equations mainly depends on conventions, rather than certain rules.
This is only my speculations on why Japanese characters aren't popular in equations.
- Conventions, across the world.
f for function.
x for unknown.
F for force,
e for Euler, etc.
g , being the successor of
f, for another function. Certain conventions above are universal, and we don't really want to use different notations for discussions in Japanese and English. When you see x = a(f), you'll get slightly confused.
- No one can read it
- There's no such thing as mathematics only for certain language community.
- Latin alphabets are more suitable for notating with a single letter.
- 「か」 for 函数 really isn't appealing for me because it loses so much information.
函（変）= 正弦(変) + 2 looks interesting, but we can't have two variables (Kanji don't have natural succ/pred). 函(変1, 変2) = 変1× 変22 might be worth discussion. There's no practical benefits, though.
- I have never been told not to Japanese characters specifically. We just learn from what are widely used and stick with them, because that's the easiest for communication.
So I believe the answer is
- It's just because it isn't conventional to do so, and there aren't many reasons to do so, actually.
- If certain conventions are established (I'm hoping 「よ」 for 米田 to be among the first), that character for that usage will be seen everywhere.
— ウニ (@unununum_1) 4 May 2017