[seliopou]

There is a reason Ξ is not used, at least as a complex variable. Say you wanna take the complex conjugate of Ξ. What would that look like? Now say you wanted to divide that by, say, Ξ. What would that look like?

It looks like a mess of lines is what it looks like.

[jacobolus]

> complex conjugate of Ξ [...] divide that by, say, Ξ

This is not a real concern; it was done by Barry Mazur as a joke, hoping to provoke Serge Lang into using his catchphrase "this notation sucks", so Mazur could present him with a prepared "This Notation Sucks" tee-shirt. See the Paul Vojta section of http://www.ams.org/notices/200605/fea-lang.pdf

[mbg721]

In before 王 becomes a thing.

[labster]

主 - U+4E3B MATHEMATICAL XI BAR WITH GRAVE ACCENT

[dustintrex]

国 U+56FD CONTAINERIZED PRODUCTION READY MATHEMATICAL XI BAR WITH SUBPRIME INDICATOR

[mbg721]

We haven't given the actuaries enough time, they'd slap an umlaut on there, and then we're good to go.