What ⨜ means
⨜ denotes the lower Darboux integral.
The Darboux lower integral comes from lower sums: each adds rectangle contributions using the lowest value a function reaches or approaches on each slice. Darboux introduced the upper and lower integrals in 1875.
Specialist real-analysis notation, precise in formulas and opaque outside mathematics.
Treat the underbar as notation: it marks the lower integral.
Don't confuse it with
4Combination ideas
1For the same function and interval, equal lower and upper integrals show it is Riemann integrable.
Questions people ask
1Why does ⨜ show as a box on some devices?
Because that device's fonts have no glyph for it. The character is still correct and still copies correctly — the box is a missing glyph, not a broken character.
How to type ⨜
Windows (Word)
Alt+XType 2A1C, then Alt+X to get ⨜.
Windows
Win+Rrun charmap for the full table, or use the emoji picker's Ω symbols tab (Win + .).
macOS
Control+Command+SpaceOpen the character viewer and search “integral with underbar”.
Linux
Ctrl+Shift+UType 2a1c, then Enter or Space.
HTML
⨜CSS
\2A1CLaTeX
With the `unicode-math` package, enter `\lowint` in LaTeX.
Codes and encoding1 code points
HTML decimal
⨜
HTML hex
⨜
CSS content
content: "\2A1C";
Each escape is closed by the next backslash; a trailing space is needed before a literal hex digit.
URL encoded
%E2%A8%9C
1
On screen
1
Code points
1
JS .length
3
UTF-8 bytes
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.