Rectangular function

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Rectangular function

The rectangular function (also known as the rectangle function, rect function, Pi function, gate function, unit pulse, or the normalized boxcar function) is defined as:[1]

\mathrm{rect}(t) = \Pi(t) = \begin{cases}
0 & \mbox{if } |t| > \frac{1}{2} \\
\frac{1}{2} & \mbox{if } |t| = \frac{1}{2} \\
1 & \mbox{if } |t| < \frac{1}{2}. \\
\end{cases}

Alternative definitions of the function define \mathrm{rect}(\pm \tfrac{1}{2}) to be 0,[2] 1,[3][4] or undefined.

Relation to the boxcar function[edit]

The rectangular function is a special case of the more general boxcar function:

\operatorname{rect}\left(\frac{t-X}{Y} \right) = u(t - (X - Y/2)) - u(t - (X + Y/2)) = u(t - X + Y/2) - u(t - X - Y/2)

Where u is the Heaviside function; the function is centered at X and has duration Y, from X-Y/2 to X+Y/2.

Another example is this: rect((t - (T/2)) / T ) goes from 0 to T, so in terms of Heaviside function u(t) - u((t-T) / T )

Fourier transform of the rectangular function[edit]

The unitary Fourier transforms of the rectangular function are[1]

\int_{-\infty}^\infty \mathrm{rect}(t)\cdot e^{-i 2\pi f t} \, dt
=\frac{\sin(\pi f)}{\pi f} = \mathrm{sinc}(\pi f),\,

using ordinary frequency f, and

\frac{1}{\sqrt{2\pi}}\int_{-\infty}^\infty \mathrm{rect}(t)\cdot e^{-i \omega t} \, dt
=\frac{1}{\sqrt{2\pi}}\cdot \frac{\mathrm{sin}\left(\omega/2 \right)}{\omega/2}
=\frac{1}{\sqrt{2\pi}} \mathrm{sinc}\left(\omega/2 \right),\,
Plot of sinc(x) function with its frequency spectral components.

using angular frequency ω, where \mathrm{sinc} is the unnormalized form of the sinc function.

Note that as long as the definition of the pulse function is only motivated by the time-domain experience of it, there is no reason to believe that the oscillatory interpretation (i.e. the Fourier transform function) should be intuitive, or directly understood by humans. However, some aspects of the theoretical result may be understood intuitively, such as the infinite bandwidth requirement incurred by the zero amplitude outside a particular time in the time-domain definition.

Relation to the triangular function[edit]

We can define the triangular function as the convolution of two rectangular functions:

\mathrm{tri} = \mathrm{rect} * \mathrm{rect}.\,

Use in probability[edit]

Viewing the rectangular function as a probability density function, it is a special case of the continuous uniform distribution with a,b=-\frac{1}{2},\frac{1}{2}. The characteristic function is:

\varphi(k) = \frac{\sin(k/2)}{k/2},\,

and its moment generating function is:

M(k)=\frac{\mathrm{sinh}(k/2)}{k/2},\,

where \mathrm{sinh}(t) is the hyperbolic sine function.

Rational approximation[edit]

The pulse function may also be expressed as a limit of a rational function:

\Pi(t) = \lim_{n\rightarrow \infty, n\in \mathbb(Z)} \frac{1}{(2t)^{2n}+1}

Demonstration of validity[edit]

First, we consider the case where |t|<\frac{1}{2}. Notice that the term (2t)^{2n} is always positive for integer n. However, 2t<1 and hence (2t)^{2n} approaches zero for large n.

It follows that:

\lim_{n\rightarrow \infty, n\in \mathbb(Z)} \frac{1}{(2t)^{2n}+1} = \frac{1}{0+1} = 1, |t|<\frac{1}{2}

Second, we consider the case where |t|>\frac{1}{2}. Notice that the term (2t)^{2n} is always positive for integer n. However, 2t>1 and hence (2t)^{2n} grows very large for large n.

It follows that:

\lim_{n\rightarrow \infty, n\in \mathbb(Z)} \frac{1}{(2t)^{2n}+1} = \frac{1}{+\infty+1} = 0, |t|>\frac{1}{2}

Third, we consider the case where |t| = \frac{1}{2}. We may simply substitute in our equation:

\lim_{n\rightarrow \infty, n\in \mathbb(Z)} \frac{1}{(2t)^{2n}+1} = \lim_{n\rightarrow \infty, n\in \mathbb(Z)} \frac{1}{1^{2n}+1} = \frac{1}{1+1} = \frac{1}{2}

We see that it satisfies the definition of the pulse function.

\therefore \mathrm{rect}(t) = \Pi(t) = \lim_{n\rightarrow \infty, n\in \mathbb(Z)} \frac{1}{(2t)^{2n}+1} = \begin{cases}
0 & \mbox{if } |t| > \frac{1}{2} \\
\frac{1}{2} & \mbox{if } |t| = \frac{1}{2} \\
1 & \mbox{if } |t| < \frac{1}{2}. \\
\end{cases}

See also[edit]

References[edit]