The Classical Greek letter for the discriminant is 'Delta' , which is shaped like a triangle with the left side being a vetrtical line and standing on a corner of the triangle.
However, 'D' or 'd' is often used in maths.
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There are no weird letters in maths. Greek mathematicians were responsible for a significant amount of the early mathematical work and they, naturally used Greek letters. Many of these letters are still used today. Just because you are not familiar with characters in the Greek alphabet does not make them "weird letters".
In ordinary everyday letters . . . pi In Greek letters . . . Π
pis
There are no books called "letters" (or epistles) in the Hebrew Old Testament. All books that are designated as "letters" are Greek, and found in the New Testament.
Athena