Googling for "Math squiggle that looks like a cursive P" is not a very elegant or convenient way of learning new symbol names.
I wish every proof or equation came with a little table that gave the English pronunciation and some context for each symbol used.
It would make it a lot easier to look up tutorials & ask questions.
And never mind that cognitive load that comes from managing the use of symbols that are the "same symbol" modulo something the typeface. Trying to read something like
"Little b equals Fraktur Bold Capital B divided by (q times Cursive Capital B) all over Gothic Italic B", blah, blah... then throw in the "weird little squiggle that looks kinda like a 'p' but not quite". It's insane.
The Greek letter xi is one of those where the capital and lowercase versions are very different. Lowercase is a bit like a curly E. Uppercase is (at least if you're writing it in a hurry) basically three horizontal lines on top of each other.
The operation called complex conjugation can be notated in two ways, but the more common one among mathematicians is to put a horizontal bar above the thing being conjugated.
So the conjugate of Xi is ... four parallel horizontal lines.
And now we divide Xi (three horizontal lines) by Xi-bar (four horizontal lines), getting: eight horizontal lines.
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-----Back in my grad school days ξ was almost always handwritten as just some sort of squiggle you'd struggle to associate with the character "ξ" unless you already knew what it was supposed to be. My classmates were very surprised that I could (can!) somehow write it as something that actually is recognizable as ξ, quickly and consistently.
(But they missed what my hand muscles do to ζ, so it all balances out.)
I spent a while browsing a Flickr group of "Greek signs", which unfortunately tended toward formality. I noticed zero examples of the joined form. There were a handful of standard Ξ, and two oddities - three examples of the letter being printed 王, and one example in which Ξ was used as the lowercase form. ( https://www.flickr.com/photos/robwallace/11888342614/in/pool... ) I guess it's easier to write than ξ is?
The letter doesn't appear to be all that common in general, unless you live in an Alexandropolis.
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Having done some more looking, I still haven't found a joined Ξ, but I have found some examples of the standard form in graffiti and handwriting (e.g. https://www.flickr.com/photos/telemax/4257624412/in/pool-gre... ). The 王 form also appears in graffiti. It's probably displacing the double-Z form.