What 𝟙 means
The subscript and the equation tell you its role.
With a set or condition as a subscript, 𝟙 denotes an indicator function: it returns 1 when the condition holds and 0 otherwise. In operator notation, it can denote the identity operator—the operation that leaves every vector unchanged.
The double-struck style, also called blackboard bold, grew from a practical problem: making bold mathematical marks on blackboards and typewriters that lacked easy boldface. Writers doubled strokes by hand or overstruck typed characters. An early printed example appeared in 1965 in Robert Gunning and Hugo Rossi's textbook on functions of complex numbers (numbers with real and imaginary parts). Familiar double-struck letters such as ℝ (real numbers) and ℕ (natural numbers) name number sets; mathematical alphabets include digits in this style as well.
Formal mathematical notation; its font style can carry meaning.
The glyph alone cannot distinguish an indicator from an identity operator; check its notation.
Part of the collection
What it means when someone sends it to you
3 readingsIndicator function: 1 when it holds, 0 otherwise.
Don't confuse it with
1
Combination ideas
1Questions people ask
1Can software change 𝟙 into 1?
Yes. Some text cleanup, called compatibility normalization (NFKC or NFKD), removes mathematical styling and leaves ordinary 1; the value survives, but the styled-symbol distinction does not.
More like this
11
Collections with 𝟙
1
How to type or find 𝟙
- Search it as
- blackboard bold 1 · indicator 1 · mathbb 1 · mathds 1 · identity operator
- Shows up in
- Anything that takes text · its shape is whatever your font draws
𝟙\1D7D9dsfont package and write \mathds{1} in math mode.Codes and encoding1 code points
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.