Abel's identity
"Abel's formula" redirects here. For the formula on difference operators, see Summation by parts.

In mathematics, Abel's identity (also called as Abel's Formula or Abel's differential equation identity) is an equation that expresses the Wronskian of two solutions of a homogeneous second-order linear ordinary differential equation in terms of a coefficient of the original differential equation.
The relation can be generalised to nth-order linear ordinary differential equations. The identity is named after the Norwegian mathematician Niels Henrik Abel.

Since Abel's identity relates the different linearly independent solutions of the differential equation, it can be used to find one solution from the other. It provides useful identities relating the solutions, and is also useful as a part of other techniques such as the method of variation of parameters. It is especially useful for equations such as Bessel's equation where the solutions do not have a simple analytical form, because in such cases the Wronskian is difficult to compute directly.

A generalisation to first-order systems of homogeneous linear differential equations is given by Liouville's formula.

Statement
Consider a homogeneous linear second-order ordinary differential equation

 

on an interval I of the real line with real- or complex-valued continuous functions p and q. Abel's identity states that the Wronskian  of two real- or complex-valued solutions  and  of this differential equation, that is the function defined by the determinant

 

satisfies the relation

 

for every point x0 in I, where C is an arbitrary constant.

Remarks
 In particular, the Wronskian  is either always the zero function or always different from zero with the same sign at every point  in . In the latter case, the two solutions  and  are linearly independent (see that article about the Wronskian for a proof).
 It is not necessary to assume that the second derivatives of the solutions  and  are continuous.
 Abel's theorem is particularly useful if , because it implies that W=const.

Proof
Differentiating the Wronskian using the product rule gives (writing  for  and omitting the argument  for brevity)

 

Solving for  in the original differential equation yields

 

Substituting this result into the derivative of the Wronskian function to replace the second derivatives of  and  gives
 

This is a first-order linear differential equation, and it remains to show that Abel's identity gives the unique solution, which attains the value  at . Since the function  is continuous on , it is bounded on every closed and bounded subinterval of  and therefore integrable, hence

is a well-defined function. Differentiating both sides, using the product rule, the chain rule, the derivative of the exponential function and the fundamental theorem of calculus, one obtains

due to the differential equation for . Therefore,  has to be constant on , because otherwise we would obtain a contradiction to the mean value theorem (applied separately to the real and imaginary part in the complex-valued case). Since , Abel's identity follows by solving the definition of  for .

Generalization
Consider a homogeneous linear th-order () ordinary differential equation

 

on an interval  of the real line with a real- or complex-valued continuous function . The generalisation of Abel's identity states that the Wronskian  of  real- or complex-valued solutions  of this th-order differential equation, that is the function defined by the determinant

 

satisfies the relation

 

for every point  in .

Direct proof
For brevity, we write  for  and omit the argument . It suffices to show that the Wronskian solves the first-order linear differential equation

because the remaining part of the proof then coincides with the one for the case .

In the case  we have  and the differential equation for  coincides with the one for . Therefore, assume  in the following.

The derivative of the Wronskian  is the derivative of the defining determinant. It follows from the Leibniz formula for determinants that this derivative can be calculated by differentiating every row separately, hence

 

However, note that every determinant from the expansion contains a pair of identical rows, except the last one. Since determinants with linearly dependent rows are equal to 0, one is only left with the last one:

 

Since every  solves the ordinary differential equation, we have

 

for every . Hence, adding to the last row of the above determinant  times its first row,  times its second row, and so on until  times its next to last row, the value of the determinant for the derivative of  is unchanged and we get

Proof using Liouville's formula
The solutions  form the square-matrix valued solution

of the -dimensional first-order system of homogeneous linear differential equations

The trace of this matrix is , hence Abel's identity follows directly from Liouville's formula.