In Latex, you get these symbols with \mathbb{}. They are most commonly used to represent to represent number sets, like the set of all integers Z (from the German), set of all natural numbers N, set of complex numbers C, set of all real numbers R, rationals Q (for “quotient”), set of quaternions H (named after Hamilton), or an unknown set F (for “field”). This explains why many of the letters in this series exist in the basic multilingual plane—because R is very commonly used, but A is not. You can find ℝ in the basic multilingual plane at U+211D.
I don’t know why people are interested in the X symbol. It’s just there to complete the alphabet. There are many other ranges like this used for writing mathematical formulas, like the range of bold letters, fraktur, script, etc.
In Latex, you get these symbols with \mathbb{}. They are most commonly used to represent to represent number sets, like the set of all integers Z (from the German), set of all natural numbers N, set of complex numbers C, set of all real numbers R, rationals Q (for “quotient”), set of quaternions H (named after Hamilton), or an unknown set F (for “field”). This explains why many of the letters in this series exist in the basic multilingual plane—because R is very commonly used, but A is not. You can find ℝ in the basic multilingual plane at U+211D.
I don’t know why people are interested in the X symbol. It’s just there to complete the alphabet. There are many other ranges like this used for writing mathematical formulas, like the range of bold letters, fraktur, script, etc.