Approximants derived by expanding a function as a ratio of two power series and determining both the numerator and denominator coefficients. Padé approximations are usually superior to Taylor series when functions contain poles, because the use of rational functions allows them to be well-represented.
The Padé approximant corresponds to the Maclaurin series. When it exists, the Padé approximant to any power series
(1)
|
is unique. If is a transcendental function, then the terms are given by the Taylor series about
(2)
|
The coefficients are found by setting
(3)
|
and equating coefficients. can be multiplied by an arbitrary constant which will rescale the other coefficients, so an additional constraint can be applied. The conventional normalization is
(4)
|
Expanding (3) gives
(5)
| |||
(6)
|
These give the set of equations
(7)
| |
(8)
| |
(9)
| |
(10)
| |
(11)
| |
(12)
| |
(13)
| |
(14)
|
where for and for . Solving these directly gives
(15)
|
where sums are replaced by a zero if the lower index exceeds the upper. Alternate forms are
(16)
|
for
(17)
| |||
(18)
|
and .
For example, the first few Padé approximants for are
(19)
| |||
(20)
| |||
(21)
| |||
(22)
| |||
(23)
| |||
(24)
| |||
(25)
| |||
(26)
| |||
(27)
| |||
(28)
| |||
(29)
| |||
(30)
| |||
(31)
| |||
(32)
| |||
(33)
| |||
(34)
|
Two-term identities include
(35)
| |||
(36)
| |||
(37)
| |||
(38)
| |||
(39)
| |||
(40)
|
where is the C-determinant. Three-term identities can be derived using the Frobenius triangle identities (Baker 1975, p. 32).
A five-term identity is
(41)
|
Cross ratio identities include
(42)
| |||
(43)
| |||
(44)
| |||
(45)
| |||
(46)
|