About Math Symbols
Unlike most collections on this site, these characters cannot be used loosely. ∑ does not mean "E" and ∈ does not mean "e" — each is an operator with a defined role, and substituting one for a letter it resembles produces text that reads as an error to anyone in a technical field. What follows is what each one actually denotes, so you can pick the right character rather than the nearest-looking one.
The Big Operators
∑ is summation — add up a series of terms. ∏ is its multiplicative counterpart, the product of a series. ∫ is the integral, and ∮ the contour integral, taken around a closed path.
√ is the square root and ∛ the cube root; both are radical signs rather than letters. ∂ denotes a partial derivative, ∇ is the nabla or del operator, and ∆ indicates a change in a quantity.
∆ Is Not the Greek Letter Delta
The character in this collection is U+2206, named INCREMENT — a mathematical operator meaning a difference or change. The Greek capital delta is a separate character, U+0394, and belongs to the Greek alphabet rather than to the operator ranges.
They look nearly identical and are not interchangeable. Search, sorting, and text processing treat them as different characters, so a word spelled with an operator where a letter belongs will not match the same word spelled correctly. If you want the letter, take it from a Greek alphabet source rather than from here.
π is the opposite case — it is the genuine Greek letter U+03C0, included because of its mathematical role.
Comparison and Set Notation
± × ÷ are plus-or-minus, multiplication, and division. The multiplication sign × is a distinct character from the letter x, and using the letter is a common and visible mistake.
≠ ≈ ≤ ≥ are not equal, approximately equal, less than or equal, and greater than or equal — single characters that render more cleanly than the two-character forms != and <=.
∈ ∉ mean is and is not a member of a set. ∅ is the empty set. ∩ ∪ are intersection and union, and ⊂ ⊃ are subset and superset. ∴ means therefore and ∵ means because — a pair used to link steps in an argument.
Rendering
All twenty-six are plain text characters that take the colour and size of the text around them, and coverage is good across normal system fonts. ∛ and ∮ are the two most likely to be missing from a stripped-down font, since they appear less often than the rest.
For notation-adjacent characters in everyday writing, the currency symbols cover the other common case where the right character is not on the keyboard. The degree sign is the one people look for most often, and how to type the degree symbol covers the shortcut on each platform.