# Partial fraction

In algebra, the **partial fraction decomposition** or **partial fraction expansion** is a procedure used to reduce the degree of *either* the numerator or the denominator of a rational function (also known as a rational algebraic fraction).

In symbols, one can use *partial fraction expansion* to change a rational function in the form

where *ƒ* and *g* are polynomials, into a function of the form

where *g*_{j} (*x*) are polynomials that are factors of *g*(*x*), and are in general of lower degree.
Thus the partial fraction decomposition may be seen as the inverse procedure of the more elementary operation of addition of algebraic fractions, that produces a single rational function with a numerator and denominator usually of high degree.
The *full* decomposition pushes the reduction as far as it will go: in other words, the factorization of *g* is used as much as possible. Thus, the outcome of a full partial fraction expansion expresses that function as a sum of fractions, where:

- the denominator of each term is a power of an irreducible (not factorable) polynomial and
- the numerator is a polynomial of smaller degree than that irreducible polynomial. To decrease the degree of the numerator directly, the Euclidean algorithm can be used, but in fact if
*ƒ*already has lower degree than*g*this isn't helpful.

The main motivation to decompose a rational function into a sum of simpler fractions is that it makes it simpler to perform linear operations on it. Therefore the problem of computing derivatives, antiderivatives, integrals, power series expansions, Fourier series, residues, and linear functional transformations of rational functions can be reduced, via partial fraction decomposition, to making the computation on each single element used in the decomposition. See e.g. partial fractions in integration for an account of the use of the partial fractions in finding antiderivatives. Just which polynomials are irreducible depends on which field of scalars one adopts. Thus if one allows only real numbers, then irreducible polynomials are of degree either 1 or 2. If complex numbers are allowed, only 1st-degree polynomials can be irreducible. If one allows only rational numbers, or a finite field, then some higher-degree polynomials are irreducible.

## Basic principles

The basic principles involved are quite simple; it is the algorithmic aspects that require attention in particular cases. On the other hand, the existence of a decomposition of a certain kind is an assumption in practical cases, and the principles should explain which assumptions are justified.

Assume a rational function *R*(*x*) = *ƒ*(*x*)/*g*(*x*) in one indeterminate *x* has a denominator that factors as

over a field *K* (we can take this to be real numbers, or complex numbers). If *P* and *Q* have no common factor, then *R* may be written as

for some polynomials *A*(*x*) and *B*(*x*) over *K*. The *existence* of such a decomposition is a consequence of the fact that the polynomial ring over *K* is a principal ideal domain, so that

for some polynomials *C*(*x*) and *D*(*x*) (see Bézout's identity).

Using this idea inductively we can write *R*(*x*) as a sum with denominators powers of irreducible polynomials. To take this further, if required, write:

as a sum with denominators powers of *F* and numerators of degree less than *F*, plus a possible extra polynomial. This can be done by the Euclidean algorithm, polynomial case. The result is the following theorem:

Let

ƒandgbe nonzero polynomials over a fieldK. Writegas a product of powers of distinct irreducible polynomials :There are (unique) polynomials

banda_{ ij}with dega_{ ij}< degp_{ i}such thatIf deg

ƒ< degg, thenb= 0.

Therefore when the field *K* is the complex numbers, we can assume that each *p*_{i} has degree 1 (by the fundamental theorem of algebra) the numerators will be constant. When *K* is the real numbers, some of the *p*_{i} might be quadratic, so in the partial fraction decomposition a quotient of a linear polynomial by a power of a quadratic will occur.

In the preceding theorem, one may replace "distinct irreducible polynomials" by "pairwise coprime polynomials that are coprime with their derivative". For example, the *p*_{i} may be the factors of the square-free factorization of *g*. When *K* is the field of the rational numbers, as it is typically the case in computer algebra, this allows to replace factorization by greatest common divisor to compute the partial fraction decomposition.

## Application to symbolic integration

For the purpose of symbolic integration, the preceding result may be refined into

Let

ƒandgbe nonzero polynomials over a fieldK. Writegas a product of powers of pairwise coprime polynomials which have no multiple root in an algebraically closed field:There are (unique) polynomials

bandc_{ ij}with degc_{ ij}< degp_{ i}such that

This reduces the computation of the antiderivative of a rational function to the integration of the last sum, with is called the *logarithmic part*, because its antiderivative is a linear combination of logarithms.

## Procedure

Given two polynomials and , where the *α*_{i} are distinct constants and deg *P* < *n*, partial fractions are generally obtained by supposing that

and solving for the *c*_{i} constants, by substitution, by equating the coefficients of terms involving the powers of *x*, or otherwise. (This is a variant of the method of undetermined coefficients.)

A more direct computation, which is strongly related with Lagrange interpolation consists in writing

where is the derivative of the polynomial .

This approach does not account for several other cases, but can be modified accordingly:

- If deg
*P*deg*Q*, then it is necessary to perform the Euclidean division of*P*by*Q*, using polynomial long division, giving*P*(*x*) =*E*(*x*)*Q*(*x*) +*R*(*x*) with deg*R*<*n*. Dividing by*Q*(*x*) this gives

- and then seek partial fractions for the remainder fraction (which by definition satisfies deg
*R*< deg*Q*).

- If
*Q*(*x*) contains factors which are irreducible over the given field, then the numerator*N*(*x*) of each partial fraction with such a factor*F*(*x*) in the denominator must be sought as a polynomial with deg*N*< deg*F*, rather than as a constant. For example, take the following decomposition over**R**:

- Suppose
*Q*(*x*) = (*x*−*α*)^{r}*S*(*x*) and*S*(*α*) ≠ 0. Then*Q*(*x*) has a zero*α*of multiplicity*r*, and in the partial fraction decomposition,*r*of the partial fractions will involve the powers of (*x*−*α*). For illustration, take*S*(*x*) = 1 to get the following decomposition:

### Illustration

In an example application of this procedure, (3*x* + 5)/(1 − 2*x*)^{2} can be decomposed in the form

Clearing denominators shows that 3*x* + 5 = *A* + *B*(1 − 2*x*). Expanding and equating the coefficients of powers of *x* gives

- 5 =
*A*+*B*and 3*x*= −2*Bx*

Solving for *A* and *B* yields *A* = 13/2 and *B* = −3/2. Hence,

### Residue method

{{#invoke:see also|seealso}}
Over the complex numbers, suppose *ƒ*(*x*) is a rational proper fraction, and can be decomposed into

Let

then according to the uniqueness of Laurent series, *a*_{ij} is the coefficient of the term (*x* − *x*_{i})^{−1} in the Laurent expansion of *g*_{ij}(*x*) about the point *x*_{i}, i.e., its residue

This is given directly by the formula

or in the special case when *x*_{i} is a simple root,

when

Note that *P*(*x*) and *Q*(*x*) may or may not be polynomials.

## Over the reals

Partial fractions are used in real-variable integral calculus to find real-valued antiderivatives of rational functions. Partial fraction decomposition of real rational functions is also used to find their Inverse Laplace transforms. For applications of **partial fraction decomposition over the reals**, see

### General result

Let *ƒ*(*x*) be any rational function over the real numbers. In other words, suppose there exist real polynomials *p*(*x*) and *q*(*x*)≠ 0, such that

By removing the leading coefficient of *q*(*x*), we may assume without loss of generality that *q*(*x*) is monic. By the fundamental theorem of algebra, we can write

where *a*_{1},..., *a*_{m}, *b*_{1},..., *b*_{n}, *c*_{1},..., *c*_{n} are real numbers with *b*_{i}^{2} - 4*c*_{i} < 0, and *j*_{1},..., *j*_{m}, *k*_{1},..., *k*_{n} are positive integers. The terms (*x* - *a*_{i}) are the *linear factors* of *q*(*x*) which correspond to real roots of *q*(*x*), and the terms (*x*_{i}^{2} + *b*_{i}*x* + *c*_{i}) are the *irreducible quadratic factors* of *q*(*x*) which correspond to pairs of complex conjugate roots of *q*(*x*).

Then the partial fraction decomposition of *ƒ*(*x*) is the following:

Here, *P*(*x*) is a (possibly zero) polynomial, and the *A*_{ir}, *B*_{ir}, and *C*_{ir} are real constants. There are a number of ways the constants can be found.

The most straightforward method is to multiply through by the common denominator *q*(*x*). We then obtain an equation of polynomials whose left-hand side is simply *p*(*x*) and whose right-hand side has coefficients which are linear expressions of the constants *A*_{ir}, *B*_{ir}, and *C*_{ir}. Since two polynomials are equal if and only if their corresponding coefficients are equal, we can equate the coefficients of like terms. In this way, a system of linear equations is obtained which *always* has a unique solution. This solution can be found using any of the standard methods of linear algebra.

## Examples

### Example 1

Here, the denominator splits into two distinct linear factors:

so we have the partial fraction decomposition

Multiplying through by *x*^{2} + 2*x* - 3, we have the polynomial identity

Substituting *x* = -3 into this equation gives *A* = -1/4, and substituting *x* = 1 gives *B* = 1/4, so that

### Example 2

After long-division, we have

Since (−4)^{2} − 4(8) = −16 < 0, *x*^{2} − 4*x* + 8 is irreducible, and so

Multiplying through by *x*^{3} − 4*x*^{2} + 8*x*, we have the polynomial identity

Taking *x* = 0, we see that 16 = 8*A*, so *A* = 2. Comparing the *x*^{2} coefficients, we see that 4 = *A* + *B* = 2 + *B*, so *B* = 2. Comparing linear coefficients, we see that −8 = −4*A* + *C* = −8 + *C*, so *C* = 0. Altogether,

The following example illustrates almost all the "tricks" one would need to use short of consulting a computer algebra system**.**

### Example 3

After long-division and factoring, we have

The partial fraction decomposition takes the form

Multiplying through by (*x* − 1)^{3}(*x*^{2} + 1)^{2} we have the polynomial identity

Taking *x* = 1 gives 4 = 4*C*, so *C* = 1. Similarly, taking *x* = *i* gives 2 + 2*i* = (*Fi* + *G*)(2 + 2*i*), so *Fi* + *G* = 1, so *F* = 0 and *G* = 1 by equating real and imaginary parts. With *C* = *G* = 1 and *F* = 0, taking *x* = 0 we get *A* - *B* + 1 - *E* - 1 = 0, thus *E* = *A* - *B*.

We now have the identity

Expanding and sorting by exponents of x we get

We can now compare the coefficients and see that

with *A* = 2 - *D* we get *A* = *D* = 1 and so *B* = 0, furthermore is *C* = 1, *E* = *A* - *B* = 1, *F* = 0 and *G* = 1.

The partial fraction decomposition of *ƒ*(*x*) is thus

Alternatively, instead of expanding, one can obtain other linear dependences on the coefficients computing some derivatives at *x=1* and at *x=i* in the above polynomial identity. (To this end, recall that the derivative at *x=a* of *(x-a) ^{m}p(x)* vanishes if

*m > 1*and it is just

*p(a)*if

*m=1*.) Thus, for instance the first derivative at

*x=1*gives

that is *8 = 2B + 8* so *B=0*.

### Example 4 (residue method)

Thus, *f*(*z*) can be decomposed into rational functions whose denominators are *z*+1, *z*-1, *z*+i, *z*-i. Since each term is of power one, -1, 1, -*i* and *i* are simple poles.

Hence, the residues associated with each pole, given by

are

respectively, and

## The role of the Taylor polynomial

The partial fraction decomposition of a rational function can be related to Taylor's theorem as follows. Let

be real or complex polynomials; assume that

that

and that

Define also

Then we have

if, and only if, for each the polynomial is the Taylor polynomial of of order at the point :

Taylor's theorem (in the real or complex case) then provides a proof of the existence and uniqueness of the partial fraction decomposition, and a characterization of the coefficients.

**Sketch of the proof:** The above partial fraction decomposition implies, for each 1 ≤ *i* ≤ *r*, a polynomial expansion

so is the Taylor polynomial of , because of the unicity of the polynomial expansion of order , and by assumption .

Conversely, if the are the Taylor polynomials, the above expansions at each hold, therefore we also have

which implies that the polynomial is divisible by

For also is divisible by , so we have in turn that is divisible by . Since we then have , and we find the partial fraction decomposition dividing by .

## Fractions of integers

The idea of partial fractions can be generalized to other integral domains, say the ring of integers where prime numbers take the role of irreducible denominators. For example:

## Notes

## References

- {{#invoke:citation/CS1|citation

|CitationClass=citation }}.

- Charles D. Miller, Margaret L. Lial, David I. Schneider:
*Fundamentals of College Algebra*. Scott & Foresman/Little & Brown Higher Education, 3rd edition 1990, ISBN 0-673-38638-4, p. 364-370

## External links

- Weisstein, Eric W., "Partial Fraction Decomposition",
*MathWorld*. - Step-by-Step Partial Fractions
- [1] Make partial fraction decompositions with Scilab.

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