## Abstract

The simplest equations with variable coefficients are considered in this research. The purpose of this study is to extend the procedure for solving the nonlinear differential equation with variable coefficients. In this case, the generalized Riccati equation is solved and becomes a basis to tackle the nonlinear differential equations with variable coefficients. The method shows that Jacobi and Weierstrass equations can be rearranged to become Riccati equation. It is also important to highlight that the solving procedure also involves the reduction of higher order polynomials with examples of Korteweg de Vries and elliptic-like equations. The generalization of the method is also explained for the case of first order polynomial differential equation.

### Keywords

- the simplest equation
- Riccati equation
- nonlinear differential equations
- reduction of polynomial

## 1. Introduction

Despite the advent of supercomputers in numerical methods, increasing activities are devoted to solving nonlinear differential equations by analytical method in recent years [1, 2, 3]. Analytical solutions have their own importance concerning the physical phenomena as they are often pave the way to the construction of right theory [4]. Many methods have been proposed concerning this important problem and generally, for the problems with constant coefficients. One of the useful methods is the method of simplest Equation [5] or for some authors, the auxiliary Equations [6]. The method is built by the utilization of the first integral of simplest nonlinear differential equations, such as Bernoulli and Riccati Equations [7]. The method had produced many new solutions of the considered nonlinear differential equations, generally with constant coefficients [8, 9].

For the more general cases, we have found that the method can be extended such as involving the solution of the nonlinear differential equations with variable coefficients. The nature of variable coefficients often arises in the equation describing the heterogenous media and composites [10] or in other cases are produced by the coordinate transformation of the partial differential Equations [11]. Those two categories are developed rapidly in recent years with the capacity of high-speed super computers which sufficient for computing nonlinear problem with complex geometries [12, 13], as sometimes desired by engineering design activities. The role of analytical solutions is as a benchmark to validate the computer algorithm with simpler geometries as it is usually performed [14].

In this chapter, the solution method of the simplest equations is different from the cases of constant coefficients except, on Bernoulli equation. Hence, we will start from Riccati equation instead of Bernoulli equation as the simplest equation to highlight the novelty of the procedure. The method is then followed by examples and conclusion.

## 2. The first integral of the simplest equations with variable coefficients

### 2.1 Riccati equation

Consider the Riccati equation with variable coefficients as follows,

Let

and is separated as

The solutions for

and

The relation for

Without loss of generality, suppose that

Rearrange Eq. (5) and integrate once,

or

The solution for

where the coefficients

### 2.2 The Jacobi and Weierstrass equations

It is interesting to know that other simplest equations can also be rearranged into the Riccati-type equations. The famous examples are Jacobi [15] and Weierstrass Equations [16], which can solve a large class of nonlinear differential equations. Let us consider Jacobi type equation with variable coefficients,

and the Weierstrass equation as follows,

Here, the reader should not be confused by the coefficients which represent different functions with the same index. Take

Concerning the search for obtaining solution of (7) and (8), the balancing principle suggests the substitution of the first order series

Performing the Riccati equation

The coefficients of polynomial are then related with the coefficients in (9) in order to determine

Hence, the first equation gives,

and the second equation is then,

The next relation produces,

Eqs. (12) and (13) are thus substituted into the fourth relation of (12) to form the third order polynomial equation in term of

which the roots will determine the solution for

Therefore, the last equation gives the expression for

In this case the higher order polynomial will be solved by reducing the order.

## 3. Reduction of higher order polynomial

Consider the sixth order polynomial equation as in the following,

First, multiply the above equation with the function

where

Note that, the coefficients

Hence, the relation for coefficients is,

The fifth coefficient relation is rearranged as,

and the roots are,

Also, the last relation is rewritten as,

Note that performing (19) into (21) will remove

which has the roots as,

Substituting back into

Therefore,

as reduced into the quartic equation the roots can be obtained by radical solution.

The procedure described by (17–25) can be applied and iterated into (16) until the polynomial equation of

Thus, following the method explained by (2–6) and (17–25), we have arrived at the solution of Jacobi and Weierstrass equations with variable coefficients.

## 4. Solution examples

### 4.1 The elliptic-like equation

As an application, consider the elliptic-like equation with forcing function,

The balance principle suggests that the solution should be in the form,

Substituting into (27) will reproduce the following expression,

The next step is to differentiate the Riccati equation once,

Substituting into Eq. (29) and it will produce the polynomial equation as in the following,

or the next step is to relate the coefficients as,

In this case, the first equation gives,

For the second equation,

Thus, provide the expression for

The third and fourth equations produce,

Substituting (32) into (31),

Replace

which then solves

### 4.2 Korteweg de Vries equation

The next example is for the Korteweg de Vries type equation,

The balancing principle with application of Riccati equation will determined the ansatz,

Performing into (35) will produce,

Performing the Riccati equation into (37) will produce the following polynomial,

From this step on, there is a little hope to solve all the coefficients as they are equal to zero. As it has also to be reduced, it is important to note that the problem of reduction here is different from the case of Jacobi equation since all the coefficients are in principle solvable in algebraic form. In this case, it is not practical to reduce the fifth order polynomial as the even highest power, i.e., as a tenth order polynomial equation. The calculation will become too tedious as the detail expression is needed in the reduced polynomial equation. The next sub section will illustrate the reduction of an odd highest power polynomial equation.

### 4.3 Reduction of fifth order polynomial

Consider Eq. (38) as follows,

Multiply by the function,

where,

Expanding the all the coefficients as,

Relate the coefficients as in the following,

The fifth equation of (41) gives the roots as,

Moving to the last equation, the functions

Substituting for

Substitute back to,

which then solves

which is reduced as,

Eq. (46) dictates that the relations,

The first equation gives,

The second equation is rewritten as,

The solution for

The third equation will produce,

Take the expression for

with,

The fourth relation of (47) will generate,

which then produce the solution of

Note that

Since only a few of the considered equation has a special polynomial to be solved by equating all the variable coefficients to zero, it is important to note that the reduction of polynomial order would be an important step. Solving all coefficients to zero often be an obstacle because the difficulty would be the same or even more than the original nonlinear ODEs. In this case, the reduction of polynomial manipulates and reduces the need for solving all coefficients.

However, it is possible not to search for the expression of variable coefficients, i.e.,

with

The definition for

Rearranging the above equation as,

Differentiating once,

Eq. (55) is a first order ODE in

## 5. Generalized method

In this section, the method of solution to the Riccati equation is extended for the class of the first order polynomial differential equation as,

The above equation can be always re-expressed as,

or

which the coefficients will be reformulated as,

In this case, we will always obtain the new coefficients

where

and is separated as,

The solutions for

The relation for

Without loss of generality, suppose that

Rearrange the above equation as,

Let

The solution for

## 6. Conclusion

In this chapter, we propose the method of the simplest or the auxiliary equation to solve the nonlinear differential equation with variable coefficients. The method is based on the solution of the generalized Riccati equation as the simplest equation. It is found that the other known simplest equations, i.e., Jacobi and Weierstrass equation, are also solved by the Riccati equation. The applications with the variable coefficients elliptic-like and Korteweg de Vries equations show that the problem of solving nonlinear differential equations with variable coefficients are simplified, especially by the reduction of the resulting polynomial equation in solving the Korteweg de Vries equation. The generalization of the method is also derived in detail.