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Blackboard bold is a style of typeface often used for certain
symbols in mathematical texts, in which certain lines of the symbol (usually
vertical, or near-vertical lines) are doubled. The symbols usually describe sets of numbers and are also referred to as double struck, although attempting to produce them by double
striking on a typewriter is unlikely to give satisfactory results. The symbols
were first introduced by the group of mathematicans known as Nicolas
Bourbaki.
In some texts, these symbols are simply shown in bold, and blackboard bold in fact originated from the attempt to write bold
letters on blackboards in a way that clearly differentiated them from non-bold letters. Wikipedia too uses ordinary bold in place
of blackboard bold, as browser support for the latter is far from universal.
TeX, the standard typesetting system for mathematical texts, does not contain direct
support for blackboard bold symbols, but the add-on AMS Fonts package by the American Mathematical Society provides this facility; a blackboard bold R is
written as \Bbb{R} in regular text and as \mathbb{R} in math mode.
In Unicode, a few of the more common blackboard bold characters
(C, H, N, P, Q, R and
Z) are encoded in the Basic Multilingual Plane (BMP). The rest, however, are encoded outside the BMP, from
U+1D538 to U+1D550 (uppercase, excluding those encoded in the BMP), U+1D552 to
U+1D56B (lowercase) and U+1D7D8 to U+1D7E1 (digits). Being outside the BMP, these are very
new and not widely supported.
The following table shows some of the more common uses of blackboard bold. The first column shows the letter as rendered by
Wikipedia's LaTeX markup system. The second column shows the Unicode codepoint. The third
column shows the symbol itself (which will only display correctly if your browser supports Unicode and has access to a suitable
font). The fourth column describes typical usage in mathematical texts.
Note that P ⊆ N ⊆ Z ⊆ Q ⊆
(A ∩ R) ⊆ R ⊆ C ⊆ H
⊆ O ⊆ S, and Q ⊆ A ⊆
C.
External links
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