Here are some facts, and my speculations.
Actual Usuages
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.
Other Alphabets
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.
Why Not?
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.
数学記号として平仮名が使われる時代が既に訪れていたのか...
pic.twitter.com/1MENyIDSwC— ウニ (@unununum_1)
4 May 2017