Characteristic Function#
The library makes heavy use of characteristic function concept and therefore, it is useful to familiarize with it.
Definition#
The characteristic function of a random variable \(x\) is the Fourier (inverse) transform of \(P^x\), where \(P^x\) is the distrubution measure of \(x\)
Properties#
\(\Phi_{x, 0} = 1\)
it is bounded, \(\left|\Phi_{x, u}\right| \le 1\)
it is Hermitian, \(\Phi_{x, -u} = \overline{\Phi_{x, u}}\)
it is continuous
characteristic function of a symmetric random variable is real-valued and even
moments of \(x\) are given by
Covolution#
The characteristic function is a great tool for working with linear combination of random variables.
if \(x\) and \(y\) are independent random variables then the characteristic function of the linear combination \(a x + b y\) (\(a\) and \(b\) are constants) is
which means, if \(x\) and \(y\) are independent, the characteristic function of \(x+y\) is the product
The characteristic function of \(ax+b\) is
Inversion#
There is a one-to-one correspondence between cumulative distribution functions and characteristic functions, so it is possible to find one of these functions if we know the other.
Continuous distributions#
The inversion formula for these distributions is given by
Discrete distributions#
In these distributions, the random variable \(x\) takes integer values. For example, the Poisson distribution is discrete. The inversion formula for these distributions is given by