Sesuatu **nombor negatif** merupakan suatu nombor yang lebih kurang daripada 0, seperti −3. Sesuatu **nombor positif** merupakan suatu nombor yang lebih besar daripada 0, seperti 3. 0 sendiri bukan negatif mahupun positif. **Nombor-nombor bukan negatif** merupakan nombor nyata yang bukan negatif (sama ada positif atau kosong). **Nombor-nombor bukan positif** merupakan nombor yang bukan positif (sama ada negatif atau kosong).

Dalam konteks nombor kompleks, *positif* adalah *nyata*, tetapi demi kejelasan seseorang juga boleh sebut "nombor nyata positif".

Fungsi signum
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Adalah mungkin untuk menentukan sesuatu fungsi sgn(*x*) pada nombor nyata yang adalah 1 bagi nombor positif, −1 bagi nombor negatif, dan 0 bagi kosong (kadangkala digelar fungsi signum):

{\displaystyle \operatorname {sgn}(x)=\left\{{\begin{matrix}-1&:x<0\\\;0&:x=0\\\;1&:x>0\end{matrix}}\right.}
Dengan ini kita ada (kecuali bagi *x*=0):

{\displaystyle \operatorname {sgn}(x)={\frac {x}{|x|}}={\frac {|x|}{x}}={\frac {d{|x|}}{d{x}}}=2H(x)-1.}
Di mana |*x*| merupakan nilai mutlak untuk *x* dan *H*(*x*) merupakan Heaviside step function. Lihat juga terbitan (matematik).

Fungsi Signum kompleks
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It is possible to define a function csgn(*x*) on the complex numbers which is 1 for positive numbers, −1 for negative numbers and 0 for zero (sometimes called the complex signum function):

{\displaystyle \operatorname {csgn} (x)=\left\{{\begin{matrix}-1&:x<0\\\;0&:x=0\\\;1&:x>0\end{matrix}}\right.}
Where the complex inequality should be interpreted as follows

{\displaystyle {\begin{cases}x>0\iff \operatorname {Re} (x)>0\vee (\operatorname {Re} (x)=0\land \operatorname {Im} (x)>0)\\x<0\iff \operatorname {Re} (x)<0\vee (\operatorname {Re} (x)=0\land \operatorname {Im} (x)<0)\\\end{cases}}}
We then have (except for *x*=0):

{\displaystyle \operatorname {csgn} (x)={\frac {x}{\sqrt {x^{2}}}}={\frac {\sqrt {x^{2}}}{x}}={\frac {d{\sqrt {x^{2}}}}{d{x}}}=2H(x)-1.}

Lihat juga
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* Nombor hipernyata
* Integer
* Rational number
* Nombor nyata
* Surreal number
* −0
* Sejarah matematik

Catatan dan rujukan
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* Templat:Ent Maseres, Francis, 1731–1824. *A dissertation on the use of the negative sign in algebra: containing a demonstration of the rules usually given concerning it; and shewing how quadratic and cubic equations may be explained, without the consideration of negative roots. To which is added, as an appendix, Mr. Machin's Quadrature of the Circle*, 1758. Quoting from Maseres' work, "If any single quantity is marked either with the sign + or the sign − without affecting some other quantity, the mark will have no meaning or significance, thus if it be said that the square of −5, or the product of −5 into −5, is equal to +25, such an assertion must either signify no more than 5 times 5 is equal to 25 without any regard for the signs, or it must be mere nonsense or unintelligible jargon."
* Templat:Ent Colva Roney-Dougal, Lecturer in Pure Mathematics at the University of St Andrews, stated this on the BBC Radio 4 "In Our Time", on Negative Numbers, 9 March 2006.
* Templat:Ent *Knowledge Transfer and Perceptions of the Passage of Time*, ICEE-2002 Keynote Address by Colin Adamson-Macedo. "*Referring again to Brahmagupta's great work, all the necessary rules for algebra, including the 'rule of signs', were stipulated, but in a form which used the language and imagery of commerce and the market place. Thus 'dhana' (= fortunes) is used to represent positive numbers, whereas 'rina' (= debts) were negative*".
* Templat:Ent Alberto A. Martinez, *Negative Math: How Mathematical Rules Can Be Positively Bent*, Princeton University Press, 2006; a history of controversies on negative numbers, mainly from the 1600s until the early 1900s.

Pautan luar
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* BBC Radio 4 series "In Our Time", on *Negative Numbers*, March 9, 2006
* Endless Examples & Exercises: *Operations With Signed Integers*

* VERY intersting and insightful concept of negative numbers