What 𝑖 means
It marks a math letter; the formula supplies its specific meaning.
Math italic usually marks a single-letter Latin variable. Under a summation sign, 𝑖 can label the running index. In complex-number notation, it commonly denotes the number whose square is 1.
Leonhard Euler chose i for 1 in a 1777 memoir, published after his death in 1794. Gauss used it in print in 1801; by 1821 it was standard notation. Separate mathematical alphabets let plain-text formulas retain distinctions that would otherwise depend on font styling. This history explains a famous use of 𝑖, but its role still comes from the formula.
Math notation: its glyph signals mathematical italic, while the formula supplies the role.
A slanted i does not automatically mean the imaginary unit; check the formula.
Part of the collection
What it means when someone sends it to you
2 readingsA running index identifies the terms being added.
Don't confuse it with
1
Questions people ask
1Why does 𝑖 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.
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How to type or find 𝑖
- Search it as
- fancy i · math font i · imaginary unit · variable i · unicode math i
- Shows up in
- Anything that takes text · its shape is whatever your font draws
𝑖\1D456i for an italic variable. This uses font styling, not necessarily the separate Unicode character.𝑖 as a numeric character reference in HTML.Codes and encoding1 code points
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.