A vinculum is a horizontal line placed over a mathematical expression, used to indicate that it is to be considered a group. Vinculum is Latin for "bond", "fetter", "chain", or "tie", which is roughly suggestive of some of the uses of the symbol.
Examples of its use include the case of a group of infinitely repeating decimal digits, for example,
It is also used in the notation of a radical to indicate the radicand whose root is being indicated. In the next case, the quantity ab + 2 is the radicand, and thus has a vinculum over it.
![\sqrt[n]{ab+2}](http://wpcontent.answers.com/math/8/0/8/8080f31cf28dcb85bfe46f8386cd7202.png)
It is also used to show the repeating terms in a periodic continued fraction. Quadratic irrational numbers are the only numbers that have these.
The vinculum is also sometimes used in Boolean algebra, where it serves to indicate a group of expressions whose logical result is to be negated, as in
It can even be used as a notation to indicate a group (bracket smaller to parenthesis):
means you will have to add b and c first and then subtract the result from a.
In particle physics, the vinculum is used to indicate antiparticles. For example, p and p are the symbols for proton and antiproton, respectively.
The vinculum should not be confused with a similar-looking vector notation, e.g. 
"vector from A to B", or 
"vector named a".
The vinculum can be typed using the combining overline (U+0305) after the number that one wishes to add it to. For example, typing ‘33.333…’ with combining overlines over the final three ‘3’s produces: ‘33.3̅3̅3̅…’.
See also
External links
- Weisstein, Eric W., "Periodic Continued Fraction" from MathWorld.
- Weisstein, Eric W., "Vinculum" from MathWorld.
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