 The definition favored by mathematicians is given by the series (Krall & Frink, 1948)

Another definition, favored by electrical engineers, is sometimes known as the reverse Bessel polynomials (See Grosswald 1978, Berg 2000).The coefficients of the second definition are the same as the first but in reverse order.For example, the third-degree Bessel polynomial is

while the third-degree reverse Bessel polynomial is

The reverse Bessel polynomial is used in the design of Bessel electronic filters.