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Positive and negative parts

Decomposition of real-valued functions

Inmathematics, the positive part of a realorextended real-valued function is defined by the formula $${\displaystyle f^{+}($$x)=\max(f(x),0)={\begin{cases}f(x)&{\text{ if }}f(x)>0\\0&{\text{ otherwise.}}\end{cases}}}

Intuitively, the graphof${\displaystyle f^{+}}$ is obtained by taking the graph of ${\displaystyle f}$, chopping off the part under the x-axis, and letting ${\displaystyle f^{+}}$ take the value zero there.

Similarly, the negative partoff is defined as $${\displaystyle f^{-}($$x)=\max(-f(x),0)=-\min(f(x),0)={\begin{cases}-f(x)&{\text{ if }}f(x)<0\\0&{\text{ otherwise}}\end{cases}}}

Note that both f⁺ and f⁻ are non-negative functions. A peculiarity of terminology is that the 'negative part' is neither negative nor a part (like the imaginary part of a complex number is neither imaginary nor a part).

The function f can be expressed in terms of f⁺ and f⁻as$${\displaystyle f=f^{+}-f^{-}.}$$

Also note that $${\displaystyle |f|=f^{+}+f^{-}.}$$

Using these two equations one may express the positive and negative parts as $${\displaystyle {\begin{aligned}f^{+}&={\frac {|f|+f}{2}}\\f^{-}&={\frac {|f|-f}{2}}.\end{aligned}}}$$

Another representation, using the Iverson bracketis$${\displaystyle {\begin{aligned}f^{+}&=[f>0]f\\f^{-}&=-[f<0]f.\end{aligned}}}$$

One may define the positive and negative part of any function with values in a linearly ordered group.

The unit ramp function is the positive part of the identity function.

Measure-theoretic properties[edit]

Given a measurable space (X, Σ), an extended real-valued function fismeasurable if and only if its positive and negative parts are. Therefore, if such a function f is measurable, so is its absolute value |f|, being the sum of two measurable functions. The converse, though, does not necessarily hold: for example, taking fas$${\displaystyle f=1_{V}-{\frac {1}{2}},}$$ where V is a Vitali set, it is clear that f is not measurable, but its absolute value is, being a constant function.

The positive part and negative part of a function are used to define the Lebesgue integral for a real-valued function. Analogously to this decomposition of a function, one may decompose a signed measure into positive and negative parts — see the Hahn decomposition theorem.

See also[edit]

• Rectifier (neural networks)
• Even and odd functions
• Real and imaginary parts

References[edit]

External links[edit]

• Positive partonMathWorld