Page 1
Classification of Second Order Partial Differential Equations
Institute of Lifelong Learning, University of Delhi 1
Lesson: Classification of Second Order Partial Differential
Equations
Course Developer: Sada Nand Prasad
College/Department: Acharya Narendra Dev College
Page 2
Classification of Second Order Partial Differential Equations
Institute of Lifelong Learning, University of Delhi 1
Lesson: Classification of Second Order Partial Differential
Equations
Course Developer: Sada Nand Prasad
College/Department: Acharya Narendra Dev College
Classification of Second Order Partial Differential Equations
Institute of Lifelong Learning, University of Delhi 2
Table of Contents:
Chapter : Classification of Second Order Partial Differential
Equations
? 1. Learning Outcomes
? 2. Introduction
? 3. Second – Order Equation in Two Independent Variables
? 4. Canonical Forms
? 5. Equations with Constant Coefficients
? 6. General Solutions
? 7. Further Simplification
? 8. The Cauchy Problem
? Summary
? Exercises
? Glossary
? References/ Further Reading
1. Learning Outcomes:
After studying this chapter, you will be able to
? classify linear second order PDEs into elliptic, parabolic and
hyperbolic types;
? reduce linear second order PDEs into canonical form;
? classify linear equation with constant coefficient and Euler equation;
? obtain the general solution of linear second order PDEs;
? further Simplification of the reduced linear second order PDEs by
introducing the new dependent variable;
? derive the Cauchy problem;
Page 3
Classification of Second Order Partial Differential Equations
Institute of Lifelong Learning, University of Delhi 1
Lesson: Classification of Second Order Partial Differential
Equations
Course Developer: Sada Nand Prasad
College/Department: Acharya Narendra Dev College
Classification of Second Order Partial Differential Equations
Institute of Lifelong Learning, University of Delhi 2
Table of Contents:
Chapter : Classification of Second Order Partial Differential
Equations
? 1. Learning Outcomes
? 2. Introduction
? 3. Second – Order Equation in Two Independent Variables
? 4. Canonical Forms
? 5. Equations with Constant Coefficients
? 6. General Solutions
? 7. Further Simplification
? 8. The Cauchy Problem
? Summary
? Exercises
? Glossary
? References/ Further Reading
1. Learning Outcomes:
After studying this chapter, you will be able to
? classify linear second order PDEs into elliptic, parabolic and
hyperbolic types;
? reduce linear second order PDEs into canonical form;
? classify linear equation with constant coefficient and Euler equation;
? obtain the general solution of linear second order PDEs;
? further Simplification of the reduced linear second order PDEs by
introducing the new dependent variable;
? derive the Cauchy problem;
Classification of Second Order Partial Differential Equations
Institute of Lifelong Learning, University of Delhi 3
2. Introduction:
In the last chapter we have derived the three fundamental equations of
mathematical physics, namely one and two - dimensional wave equation,
one - dimensional heat conduction equation/diffusion equation and
Laplace’s equation and mentioned that these equations are of hyperbolic,
parabolic and elliptic type. Here, in this chapter we will explain in detail,
about the classification of these second order partial differential equations
with variable and constant coefficients and will find the general solution
after reducing these equations to their respective canonical form.
To begin with, we have in this chapter described the second order partial
differential equations (PDEs) in two independent variables and classified
linear PDEs of second order into elliptic, parabolic and hyperbolic types.
3. Second – Order Partial Differential Equation in Two
Independent Variables :
It is seen that a large number of PDEs arising in the study of applied
mathematics with special reference to biological, physical and engineering
applications, can be treated as a particular case of the most general form
of a linear, second order PDE in two independents of the form
2 2 2
22
,
z z z z z
a b c d e f z g
x x y y x y
? ? ? ? ?
? ? ? ? ? ?
? ? ? ? ? ?
or,
,
xx xy yy x y
az bz cz d z ez f z g ? ? ? ? ? ? (1)
where a, b, c, d, e, f, and g are functions of the independent variables x
and y and do not vanish simultaneously.
Value Addition: Do you know?
From coordinate geometry, we know that general equation of second
degree in two variables
22
0, ax bxy cy d x ey f ? ? ? ? ? ?
represents, ellipse if b
2
– 4 a c < 0,
parabola if b
2
– 4 a c = 0 or, (2)
hyperbola if b
2
– 4 a c > 0.
The classification of second order partial differential equations (3.1), is
suggested by the classification of the above equation (2), and based upon
the possibilities of transforming equation (1) to canonical form at any
point (x
0
, y
0
) by suitable coordinate transformation. Therefore, an
equation at any point (x
0
, y
0
) is called
elliptic if b
2
(x
0
, y
0
)– 4 a(x
0
, y
0
) c(x
0
, y
0
) < 0,
parabolic if b
2
(x
0
, y
0
)– 4 a(x
0
, y
0
) c(x
0
, y
0
) = 0 or, (3)
hyperbolic if b
2
(x
0
, y
0
)– 4 a(x
0
, y
0
) c(x
0
, y
0
) > 0.
Page 4
Classification of Second Order Partial Differential Equations
Institute of Lifelong Learning, University of Delhi 1
Lesson: Classification of Second Order Partial Differential
Equations
Course Developer: Sada Nand Prasad
College/Department: Acharya Narendra Dev College
Classification of Second Order Partial Differential Equations
Institute of Lifelong Learning, University of Delhi 2
Table of Contents:
Chapter : Classification of Second Order Partial Differential
Equations
? 1. Learning Outcomes
? 2. Introduction
? 3. Second – Order Equation in Two Independent Variables
? 4. Canonical Forms
? 5. Equations with Constant Coefficients
? 6. General Solutions
? 7. Further Simplification
? 8. The Cauchy Problem
? Summary
? Exercises
? Glossary
? References/ Further Reading
1. Learning Outcomes:
After studying this chapter, you will be able to
? classify linear second order PDEs into elliptic, parabolic and
hyperbolic types;
? reduce linear second order PDEs into canonical form;
? classify linear equation with constant coefficient and Euler equation;
? obtain the general solution of linear second order PDEs;
? further Simplification of the reduced linear second order PDEs by
introducing the new dependent variable;
? derive the Cauchy problem;
Classification of Second Order Partial Differential Equations
Institute of Lifelong Learning, University of Delhi 3
2. Introduction:
In the last chapter we have derived the three fundamental equations of
mathematical physics, namely one and two - dimensional wave equation,
one - dimensional heat conduction equation/diffusion equation and
Laplace’s equation and mentioned that these equations are of hyperbolic,
parabolic and elliptic type. Here, in this chapter we will explain in detail,
about the classification of these second order partial differential equations
with variable and constant coefficients and will find the general solution
after reducing these equations to their respective canonical form.
To begin with, we have in this chapter described the second order partial
differential equations (PDEs) in two independent variables and classified
linear PDEs of second order into elliptic, parabolic and hyperbolic types.
3. Second – Order Partial Differential Equation in Two
Independent Variables :
It is seen that a large number of PDEs arising in the study of applied
mathematics with special reference to biological, physical and engineering
applications, can be treated as a particular case of the most general form
of a linear, second order PDE in two independents of the form
2 2 2
22
,
z z z z z
a b c d e f z g
x x y y x y
? ? ? ? ?
? ? ? ? ? ?
? ? ? ? ? ?
or,
,
xx xy yy x y
az bz cz d z ez f z g ? ? ? ? ? ? (1)
where a, b, c, d, e, f, and g are functions of the independent variables x
and y and do not vanish simultaneously.
Value Addition: Do you know?
From coordinate geometry, we know that general equation of second
degree in two variables
22
0, ax bxy cy d x ey f ? ? ? ? ? ?
represents, ellipse if b
2
– 4 a c < 0,
parabola if b
2
– 4 a c = 0 or, (2)
hyperbola if b
2
– 4 a c > 0.
The classification of second order partial differential equations (3.1), is
suggested by the classification of the above equation (2), and based upon
the possibilities of transforming equation (1) to canonical form at any
point (x
0
, y
0
) by suitable coordinate transformation. Therefore, an
equation at any point (x
0
, y
0
) is called
elliptic if b
2
(x
0
, y
0
)– 4 a(x
0
, y
0
) c(x
0
, y
0
) < 0,
parabolic if b
2
(x
0
, y
0
)– 4 a(x
0
, y
0
) c(x
0
, y
0
) = 0 or, (3)
hyperbolic if b
2
(x
0
, y
0
)– 4 a(x
0
, y
0
) c(x
0
, y
0
) > 0.
Classification of Second Order Partial Differential Equations
Institute of Lifelong Learning, University of Delhi 4
Example 1. Consider the equation,
2
0,
xx yy
z x z ?? , comparing this equation
with equation (1), we get, a = 1, b = 0, c = x
2
, and therefore, b
2
- 4ac <
0. Thus, the given equation is elliptic. Similarly, we can say that the
Laplace’s equation derived in the last chapter 0,
xx yy
uu ?? is elliptic.
Example 2. Consider the equation, 2 0,
xx xy yy
z z z ? ? ? , comparing this
equation with equation (1), we get, a = 1, b = 2, c = 1, and therefore,
b
2
- 4ac = 0. Thus, the given equation is parabolic. Similarly, we can say
that the one – dimensional heat equation / diffusion equation derived in
the last chapter
t xx
T KT ? is hyperbolic.
Example 3. Consider the equation,
2
xx yy
z x z ? , comparing this equation
with equation (1), we get, a = 1, b = 0, c = x
2
, and therefore, b
2
- 4ac =
4x
2
> 0. Thus, the given equation is hyperbolic. Similarly, we can say
that the one - dimensional wave equation derived in the last chapter
2
tt xx
u c u ? is hyperbolic.
It may also be remarked here that an equation can be of mixed type,
depending upon its coefficients.
Example 4. The equation given by
2
,
xx yy
xz z x ?? can be classified as
elliptic if, x > 0, parabolic, if x = 0, or hyperbolic if x < 0 as b
2
- 4ac = - 4
x.
Now, we shall show that by a suitable change in the independent
variables, we can reduce any equation of type (1) to one of the three
standard or canonical forms. Let us suppose we change the variables x
and y to u and v, respectively where
u = u (x, y), v = v(x, y). (4)
We will also assume that u and v are twice continuously differentiable and
in the region under consideration the Jacobian
0.
xy
xy
uu
vv
?
Then for the system (4), we can determine x and y uniquely. Suppose x
and y are twice continuously differentiable functions of u and v. Then, we
have,
Page 5
Classification of Second Order Partial Differential Equations
Institute of Lifelong Learning, University of Delhi 1
Lesson: Classification of Second Order Partial Differential
Equations
Course Developer: Sada Nand Prasad
College/Department: Acharya Narendra Dev College
Classification of Second Order Partial Differential Equations
Institute of Lifelong Learning, University of Delhi 2
Table of Contents:
Chapter : Classification of Second Order Partial Differential
Equations
? 1. Learning Outcomes
? 2. Introduction
? 3. Second – Order Equation in Two Independent Variables
? 4. Canonical Forms
? 5. Equations with Constant Coefficients
? 6. General Solutions
? 7. Further Simplification
? 8. The Cauchy Problem
? Summary
? Exercises
? Glossary
? References/ Further Reading
1. Learning Outcomes:
After studying this chapter, you will be able to
? classify linear second order PDEs into elliptic, parabolic and
hyperbolic types;
? reduce linear second order PDEs into canonical form;
? classify linear equation with constant coefficient and Euler equation;
? obtain the general solution of linear second order PDEs;
? further Simplification of the reduced linear second order PDEs by
introducing the new dependent variable;
? derive the Cauchy problem;
Classification of Second Order Partial Differential Equations
Institute of Lifelong Learning, University of Delhi 3
2. Introduction:
In the last chapter we have derived the three fundamental equations of
mathematical physics, namely one and two - dimensional wave equation,
one - dimensional heat conduction equation/diffusion equation and
Laplace’s equation and mentioned that these equations are of hyperbolic,
parabolic and elliptic type. Here, in this chapter we will explain in detail,
about the classification of these second order partial differential equations
with variable and constant coefficients and will find the general solution
after reducing these equations to their respective canonical form.
To begin with, we have in this chapter described the second order partial
differential equations (PDEs) in two independent variables and classified
linear PDEs of second order into elliptic, parabolic and hyperbolic types.
3. Second – Order Partial Differential Equation in Two
Independent Variables :
It is seen that a large number of PDEs arising in the study of applied
mathematics with special reference to biological, physical and engineering
applications, can be treated as a particular case of the most general form
of a linear, second order PDE in two independents of the form
2 2 2
22
,
z z z z z
a b c d e f z g
x x y y x y
? ? ? ? ?
? ? ? ? ? ?
? ? ? ? ? ?
or,
,
xx xy yy x y
az bz cz d z ez f z g ? ? ? ? ? ? (1)
where a, b, c, d, e, f, and g are functions of the independent variables x
and y and do not vanish simultaneously.
Value Addition: Do you know?
From coordinate geometry, we know that general equation of second
degree in two variables
22
0, ax bxy cy d x ey f ? ? ? ? ? ?
represents, ellipse if b
2
– 4 a c < 0,
parabola if b
2
– 4 a c = 0 or, (2)
hyperbola if b
2
– 4 a c > 0.
The classification of second order partial differential equations (3.1), is
suggested by the classification of the above equation (2), and based upon
the possibilities of transforming equation (1) to canonical form at any
point (x
0
, y
0
) by suitable coordinate transformation. Therefore, an
equation at any point (x
0
, y
0
) is called
elliptic if b
2
(x
0
, y
0
)– 4 a(x
0
, y
0
) c(x
0
, y
0
) < 0,
parabolic if b
2
(x
0
, y
0
)– 4 a(x
0
, y
0
) c(x
0
, y
0
) = 0 or, (3)
hyperbolic if b
2
(x
0
, y
0
)– 4 a(x
0
, y
0
) c(x
0
, y
0
) > 0.
Classification of Second Order Partial Differential Equations
Institute of Lifelong Learning, University of Delhi 4
Example 1. Consider the equation,
2
0,
xx yy
z x z ?? , comparing this equation
with equation (1), we get, a = 1, b = 0, c = x
2
, and therefore, b
2
- 4ac <
0. Thus, the given equation is elliptic. Similarly, we can say that the
Laplace’s equation derived in the last chapter 0,
xx yy
uu ?? is elliptic.
Example 2. Consider the equation, 2 0,
xx xy yy
z z z ? ? ? , comparing this
equation with equation (1), we get, a = 1, b = 2, c = 1, and therefore,
b
2
- 4ac = 0. Thus, the given equation is parabolic. Similarly, we can say
that the one – dimensional heat equation / diffusion equation derived in
the last chapter
t xx
T KT ? is hyperbolic.
Example 3. Consider the equation,
2
xx yy
z x z ? , comparing this equation
with equation (1), we get, a = 1, b = 0, c = x
2
, and therefore, b
2
- 4ac =
4x
2
> 0. Thus, the given equation is hyperbolic. Similarly, we can say
that the one - dimensional wave equation derived in the last chapter
2
tt xx
u c u ? is hyperbolic.
It may also be remarked here that an equation can be of mixed type,
depending upon its coefficients.
Example 4. The equation given by
2
,
xx yy
xz z x ?? can be classified as
elliptic if, x > 0, parabolic, if x = 0, or hyperbolic if x < 0 as b
2
- 4ac = - 4
x.
Now, we shall show that by a suitable change in the independent
variables, we can reduce any equation of type (1) to one of the three
standard or canonical forms. Let us suppose we change the variables x
and y to u and v, respectively where
u = u (x, y), v = v(x, y). (4)
We will also assume that u and v are twice continuously differentiable and
in the region under consideration the Jacobian
0.
xy
xy
uu
vv
?
Then for the system (4), we can determine x and y uniquely. Suppose x
and y are twice continuously differentiable functions of u and v. Then, we
have,
Classification of Second Order Partial Differential Equations
Institute of Lifelong Learning, University of Delhi 5
. . . .
. . . .
2
. . . .
2
z z u z v
z z u z v
x u x v x
x u x v x
z z u z v
z z u z v
y u y v y
y u y v y
z z u v z u z v
z
xx
x x u x v x u x v x
x
? ? ? ? ? ?
? ? ? ? ? ?
? ? ? ? ? ?
? ? ? ? ? ?
? ? ? ? ?
? ? ? ? ?
? ? ? ? ?
? ? ? ? ?
? ? ? ? ?
? ? ? ? ?
? ? ? ? ? ? ? ? ? ? ?
? ? ? ? ?
? ? ? ? ? ? ? ? ? ?
?
22
2 2 2 2 2
2
2 . 2 2 2
2 2 2 2
2
2
. . . .
2
z u z u v z v z u z v
x u v x x x u v
u v x x
z u z u v z v z u z v
uu x uv x x vv x u x v x
z z u v z u z v
z
xy
x y x y u x v x u y v y
z
? ? ? ?
? ? ? ?
? ? ? ?
? ? ? ?
? ? ? ? ??
? ? ? ? ??
?? ? ? ? ?
?? ? ? ? ?
? ? ? ? ? ? ? ? ? ? ?
? ? ? ? ?
? ? ? ? ? ? ? ?
? ? ? ?
? ? ? ? ?
? ? ? ? ? ? ? ? ? ? ?
? ? ? ? ?
? ? ? ? ? ? ? ? ? ? ? ?
?
?
?
2 2 2 2
2 . 2
2
. . . .
2
u u z u v u v z v v z u z v
x y u v x y y x x y u y x v y x
uv
z u u z u v u v z v v z u z v
uu x y uv x y y x vv x y u xy v xy
z z u v z u z v
z
yy
y y u y v y u y v
y
??
??
??
??
??
??
??
? ? ? ?
? ? ? ?
? ? ? ?
? ? ? ?
? ? ? ? ? ? ? ? ? ? ? ? ? ?
? ? ? ? ?
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
?
? ? ? ? ? ?
? ? ? ? ? ? ? ? ? ? ?
? ? ? ? ?
? ? ? ? ? ? ? ? ? ?
?
22
2 2 2 2 2
2
2 . 2 2 2
2 2 2 2
2
y
z u z u v z v z u z v
y u v y y y u v
u v y y
z u z u v z v z u z v
uu y uv y y vv y u y v y
??
??
??
??
? ? ? ?
? ? ? ?
? ? ? ?
? ? ? ?
? ? ? ? ? ? ? ? ? ? ?
? ? ? ? ?
? ? ? ? ? ? ? ?
? ? ? ?
? ? ? ? ?
(5)
Substituting all these values in equation (1), we get,
* * * * * * *
uu uv vv u v
a z b z c z d z e z f z g ? ? ? ? ? ?
where
? ?
* 2 2
*
* 2 2
*
*
*
*
,
2 2 ,
,
,
,
,
.
x x y y
x x x y y x y y
x x y y
xx xy yy x y
xx xy yy x y
a au bu u cu
b au v b u v u v cu v
c av bv v cv
d au bu cu du eu
e av bv cv dv ev
ff
gg
? ? ?
? ? ? ?
? ? ?
? ? ? ? ?
? ? ? ? ?
?
?
(6)
The classification of the second order PDE (1) depends on the coefficients
a(x, y), b(x, y), and c(x, y) at any point (x, y). So, we can write equation
(1) and equation (2) as
? ?
, , , ,
xx xy yy x y
az bz cz h x y z z z ? ? ? (7)
and
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