2017, Volume 4, e3741 ISSN Online: 2333-9721 ISSN Print: 2333-9705

**Laplace Inverse Transform for Functions of **

**Type nth Root of a Product of Linear Factors **

**Uriel Salmeron-Rodriguez**

Centro de Ingenieria y Desarrollo Industrial (CIDESI), Queretaro, Mexico

**Abstract **

In this work, we present four results for the Laplace inverse transform of functions that involve the nth root of a product of linear factors. In order to find the Laplace inverse transform, we considered a branch cut for the nth root and a region of suitable integration, to avoid the branching points. Due to that, the solution is in terms of integrals, we easily approach this solution for some specific parameters.

**Subject Areas **

Mathematical Analysis, Mathematical Logic and Foundation of Mathematics

**Keywords **

Laplace Inverse Transform, Multivalued Functions, Integration Contour, Stehfest Numerical Method

**1. Introduction **

Modeling phenomena using partial differential equations are attractive to physicists, mathematicians, engineers, etc., this is due to many phenomena such as diffusion of particles, heat diffusion, fluid flow behavior on porous media, among others, are modeled with this type of differential equations (see

[1]-[7]). However, when we use the Laplace transform to find the solution of these models, it is likely to find multivalued functions, so the Laplace inverse transform is commonly solved by numerical methods, for example, in [8] [9]

studied models that predict the fluid flow on porous media, they used the Laplace transform and found multivalued functions of the type ., and to give the solution through Laplace inverse transform they used the Stehfest numerical method. The Stehfest numerical method has restrictions for functions that have discontinuities [10], so it is more advisable to find the exact solution. In some

How to cite this paper: Salmeron-Rodri- guez, U. (2017) Laplace Inverse Transform for Functions of Type nth Root of a Product of Linear Factors. Open Access Library Jour- nal, 4: e3741.

https://doi.org/10.4236/oalib.1103741 Received: June 13, 2017

Accepted: July 3, 2017 Published: July 6, 2017

Copyright © 2017 by author and Open Access Library Inc.

This work is licensed under the Creative Commons Attribution International License (CC BY 4.0).

http://creativecommons.org/licenses/by/4.0/

cases, it is possible to give the exact solution [11], but to obtain the exact solution of these models, a deeper study of the Laplace inverse transform of the multivalued functions found is necessary. In this work, we study Laplace inverse transform for functions that involve the nth root of a product of linear factors. It is divided into two sections as follows: In the first section, we use the Laplace inverse transform defined as

### ( )

1### ( )

e d ,

2π
*i* _{st}*i*

*f t* *F s* *s*

*i*

δ δ

+ ∞ − ∞

=

_{∫}

### (1)

to find the function

*f t*

### ( )

. Here### δ

>0 and*F s*

### ( )

involve to### (

0### )(

1### )(

2### ) (

1### )(

### )

,*n*

*k* *k*

*s*+*a* *s*+*a* *s*+*a* *s*+*a*_{−} *s*+*a*

where n is positive integer and *a*0 < <*a*1 *a*2<<*ak* are real positive. The
function

*f t*

### ( )

is represented in terms of integrals that are easily approximated numerically. To find this function, we consider the branch cut and the integration contour of Figure 1. [image:2.595.259.486.337.722.2]In section two, We give analytical examples and also numerically solve the function

*f t*

### ( )

for some particular cases and compare versus the Stehfestnumerical method. We also show that the Stehfest numerical method does not approximate well to the exact solution near the discontinuities.

**2. Theorems **

In this section, we propose and prove four theorems associated with the Laplace
inverse transform for multivalued functions that involve *n*.. It should be
mentioned that in particular Theorem 4 can be used to find the solution of some
differential equations that model the fluid flow on porous media, for example

[11].

Theorem 1. Suppose *n*∈+, *t*>0, 0=*a*0<*a*1<*a*2<<*ak*<*ak*+1= ∞ and

### ( )

### (

0### )(

1### )(

2### ) (

1### )(

### )

1

,
*n*

*k* *k*

*F s* *s* *a* *s* *a* *s* *a* *s* *a* *s* *a*

*s* −

= + + + + +

then the Laplace inverse transform of the function

*F s*

### ( )

is given by### ( )

### (

### )

1### ( )

0

1 π

1 1

sin e d ,

π

*j*
*j*

*k* _{a}

*xt*
*j*
*a*

*j*

*j*

*f t* *u* *x* *x*

*n* *x*

+ − =

+

= _{} _{}

### ∑

### ∫

### (2)

where

### ( )

*n*

### (

0### )(

1### )

### (

### )(

1### )

### (

1### )(

### )

.*j* *j* *j* *k* *k*

*u* *x* = *x*−*a* *x*−*a* *x*−*a* − +*x* *a*+ − +*x* *a*− − +*x* *a*

Proof. Using the Laplace inverse transform, we have

### ( )

1### ( )

e d .

2π

*i* _{sx}

*i*

*f t* *F s* *s*

*i*

δ δ

+ ∞ − ∞

=

_{∫}

As *a*0,− −*a*1, *a*2,,−*ak* are branch points of the function

*F s*

### ( )

, then we consider the region of the Figure 1 and the branch cut### − <

### π

*Arg s*

### ( )

### <

### π

for thenth root. Using Cauchy Theorem is found

### ( )

### ( )

## (

## )

## (

11 11 1 1 1 0 2## )

1

e d

2π 1 2π

1

. 2π

*k* *k*

*k* *k* *k* *k*

*i* _{sx}*i*

*f t* *F s* *s*

*i*

*i*

*i*

δ δ

γ

γ γ γ γ γ γ

+ +

+ +

+ ∞ − ∞

′ ′

Γ Γ Γ Γ

′ ′

=

= − + + + + +

− + + + + + +

### ∫

### ∫ ∫

### ∫

### ∫

### ∫

### ∫ ∫

### ∫ ∫ ∫

### ∫

### (3)

It is easy to prove that

0 0

γ =

### ∫

when →0. In addition also 0*j*

γ →

### ∫

to1, 2, ,

*j*= *k* when →0, this is because if *s* *aj* e*i*

θ

= − + with 0< <θ π then

( )

### (

### ) (

### ) (

### )

### ( )

π

0 0

e

d ,

*j*
*j*

*a* *t*

*n*

*j* *j* *j* *j* *k*

*j*

*a* *a* *a* *a* *a* *a*

*a*

γ θ

− +

< − + + − + + − + +

− +

### ∫

### ∫

thus 0

*j*

γ →

### ∫

when →0. Analogously if*s*

*aj*e

*i*

θ

= − + with − < <π θ 0 we

prove that 0

*j*

γ ′ →

### ∫

when →0.For

1
*k*

γ+

### ∫

, we obtain( )

_{(}

_{)(}

_{) (}

_{)}

1
cos

0 1

e d ,

*k*

*R* _{t n}

*k*

*R* *a* *R* *a* *R* *a*

π _{θ}

γ+ < α + + +

### θ

as *t*>0 and cosθ is negative in

### (

### π 2,π

### )

, then e*Rt*cos( )θ →0 when

*R*→ ∞, thus

1

0

*k*

γ+ →

### ∫

it is for δ small enough. Similarly for2
*k*

γ+

### ∫

.For integrals over Γ*j*+1 and over Γ′*j*+1 with *j*=0,1, 2,,*k* we analyze as

follows: If π

e*i*

*s*=*x* = −*x* with *x*∈

### (

*a aj*,

*j*+1

### )

we obtain### (

### )

### (

### )

### (

### )

π 1 1 0 1e for 0,1, , ,

e for 1, 2, , .

*i*
*n*
*n*
*n* *k*
*k* *i*
*n*
*n*
*k*

*x* *a* *k* *j*

*s* *a*

*x* *a* *k* *j* *j* *k*

− =
+ =
_{− +} _{= +} _{+}

_{ }

(4)
Using Equation (4) we find

### ( )

### (

### )(

### )

### (

### )(

### )

### (

### )

### ( ) (

### )

### (

### ) (

### )

### (

### )

### (

### )

### (

### )

### (

### )

### (

### )

( )

### ( )

1 0 1 1

1 1 1

1 1

1 1

π π π 0 0

1 1 1

1 1

1 1

1π

e e e e e

e ,

*n*

*j* *j* *k*

*n* *n* *n*

*n* *n*

*j* *j* *k*

*i* *i* *i* *i* *i*

*n* *n* *n*

*n*
*n*

*n* *n* *n* *n* *n*

*j* *j* *k*

*j* *i*
*n*

*j*

*u x* *s* *a* *s* *a* *s* *a* *s* *a* *s* *a*

*s* *s* *a* *s* *a* *s* *a* *s* *a*

*x* *x* *a* *x* *a* *x* *a* *x* *a*

*u* *x*
+
+
+
+
= + + + + +
= + + + +
= − − − + − +
=
then

### ( )

1### ( )

1 1 1 1

1 1

e d e d .

*j* *j*

*j* *j* *j*

*a* _{xt}*a* _{xt}

*a* _{x}*u x* *x* *a* _{x}*u x* *x*

+

+ +

− −

Γ = = −

### ∫

### ∫

### ∫

### (5)

On the other hand, if _{e} *i*π

*s*=*x* − = −*x* con *x*∈

### (

*a aj*,

*j*+

_{1}

### )

we obtain### ( )

### (

### )(

### )

### (

### )(

### )

### (

### )

( )1π### ( )

2 0 1 1 e ,

*j* *i*

*n*
*n*

*j* *j* *k* *j*

*u* *x* *s* *a* *s* *a* *s* *a* *s* *a* *s* *a* *u* *x*

− + + = + + + + + = then

### ( )

1 1 2 1e d .

*j*

*j* *j*

*a* _{xt}

*a* _{x}*u* *x* *x*

+ + − ′ Γ =

### ∫

### ∫

### (6)

Thus, for Equation (5) and Equation (6) we find

### (

### )

1_{( )}

1 1

1 π 1

2 sin *j* e d .

*j* *j* *j*

*a* _{xt}

*j*
*a*

*j*

*i* *u* *x* *x*

*n* *x*
+
+ +
−
′
Γ Γ
+
+ = −

### ∫

### ∫

### ∫

Finally, adding all integrals and replacing in Equation (3), we obtain

### ( )

### (

### )

1### ( )

0

1 π

1 1

sin e d .

π
*j*
*j*
*k* _{a}*xt*
*j*
*a*
*j*
*j*

*f t* *u* *x* *x*

*n* *x*
+ −
=
+
=

### ∑

### ∫

Theorem 2. Suppose *n*∈+, *t*>0, 0=*a*0<*a*1<*a*2<<*ak*<*ak*+1= ∞ and

### ( )

### (

0### )(

1### )(

2### ) (

1### )(

### )

1

,
*n*

*k* *k*

*F s*

*s* *a* *s* *a* *s* *a* *s* *a* − *s* *a*

=

+ + + _{} + +

then the Laplace inverse transform of the function

*F s*

### ( )

is given by### ( )

### (

### )

1### ( )

0

1 π 1

sin e d ,

π
*j*
*j*
*k*
*a* _{xt}*j*
*a*
*j*
*j*

*f t* *u* *x* *x*

### ( )

### (

0### )(

1### )

### (

### )(

1### )

### (

1### )(

### )

1

.
*j*

*n*

*j* *j* *k* *k*

*u* *x*

*x* *a* *x* *a* *x* *a* *x* *a*+ *x* *a*− *x* *a*

=

− − _{} − − + _{} − + − +

Proof. Again we use the region Figure 1. In analogy with the proof of the

previous Theorem we have

0

0

γ →

### ∫

, 0*j*

γ →

### ∫

and 0*j*

γ ′ →

### ∫

when →0 for1, 2, ,

*j*= *k* and also

1

0

*k*

γ+ →

### ∫

and2

0

*k*

γ+ →

### ∫

when*R*→ ∞.

On the other hand we have

### ( )(

### )

1### ( )

1 1 1 1

d e d ,

*j* *j*

*j* *j* *j*

*a* _{xt}*a* _{xt}

*a* *e* *u x* *x* *a* *u x* *x*

+

+ +

− −

Γ = − =

### ∫

### ∫

### ∫

### ( )

1

1 2

e d ,

*j*

*j* *j*

*a* _{xt}

*a* *u* *x* *x*

+

+

− ′

Γ = −

### ∫

### ∫

where

### ( )

( )1π### ( )

### ( )

( )1π### ( )

1 e and 2 e ,

*j* *i* *j* *i*

*n* *n*

*j* *j*

*u x* *u* *x* *u* *x* *u* *x*

− + +

= =

then

### (

### )

1_{( )}

1 1

1 π

2 sin *j* e d .

*j* *j* *j*

*a* _{xt}

*j*
*a*
*j*

*i* *u* *x* *x*

*n*
+
+ +
−
′
Γ Γ
+
+ = −

### ∫

### ∫

### ∫

Therefore, we add all the integrals so we find the result.

Theorem 3. Suppose *n m*, ∈+, *t*>0, 0=*a*0<*a*1<*a*2<<*ak* <*ak*+1= ∞

and

### ( )

### (

### )(

### )(

### ) (

### )(

### )

### (

### )(

### )(

### ) (

### )(

### )

0 2 4 3 1

1 3 5 2

1
,
*n*
*k* *k*
*m*
*k* *k*

*s* *a* *s* *a* *s* *a* *s* *a* *s* *a*

*F s*

*s* *s* *a* *s* *a* *s* *a* *s* *a* *s* *a*

− − − + + + + + = + + + + +

then the Laplace inverse transform of the function

*F s*

### ( )

is given by### ( )

### (

### )

### ( )

### (

### )

### (

### )

_{( )}

2 1
2
2 2
2 1
2
2
0
2
2 1
0
1 π
1 π 1

sin e d

π

1 π 1 π

1 1

sin e d ,

π
*j*
*j*
*j*
*j*
*k*
*a* _{xt}*j*
*a*
*j*
*k*
*a* _{xt}*j*
*a*
*j*
*j* *j*

*f t* *u* *x* *x*

*n* *m* *x*

*j* *j*

*u* *x* *x*

*n* *m* *x*

+
+
+
−
=
−
+
=
+
= _{} − _{}
+ +
+ _{} − _{}

### ∑

### ∫

### ∑

### ∫

### (8) where

### ( ) (

### ) (

1### )

1### (

### ) (

1### )

1### (

### ) (

1### )

10 *n* 1 *m* *n* 1 *m* 1 *n* *m*.

*j* *j* *j* *k* *k*

*u* *x* = *x*−*a* *x*−*a* − *x*−*a* − +*x* *a*+ − − +*x* *a*− − +*x* *a* −

Note that

### (

### ) (

1### )

11

*n* *m*

*j* *j*

*x*−*a* − +*x* *a*+ − can change by

### (

### ) (

### )

1 1

1

*m* *n*

*j* *j*

*x*−*a* − − +*x* *a*+

when k is even or odd.

Proof. The proof is analogous to the previous theorems.

Theorem 4. Suppose *t*>0, *r*>0, 0=*a*0<*a*1<*a*2<<*ak*<*ak*+1= ∞ and

### ( )

### (

### (

### )(

### )(

### ) (

### )(

### )

### )

### (

### )(

### )(

### ) (

### )(

### )

### (

### )

0 0 1 2 1

0 0 1 2 1

1

,

*k* *k*

*k* *k*

*K* *r* *s* *a* *s* *a* *s* *a* *s* *a* *s* *a*

*F s*

*s K* *s* *a* *s* *a* *s* *a* *s* *a* *s* *a*

− − + + + + + = + + + + +

where *K*0 is the modified Bessel function of order 0 then the Laplace inverse

### ( )

2 1### ( )

2### ( )

2 2 2 1

1 2

2 2 2 1

1 1

1 1 1

1 e , d e , d

π

*j* *j*

*j* *j*

*k*

*L*

*a* _{xt}*a* _{xt}

*j* *j*

*a* *a*

*j* *j*

*f t* *r x* *x* *r x* *x*

*x* *x*
−
− −
+
− −
− −
= =

= +

### ∑

_{∫}

Ψ +### ∑

_{∫}

Ψ
### (9)

where

for even, 2

1 for odd,

2
*k*
*k*
*L*
*k*
*k*
=
+
_{ }

### ( )

### ( )

### (

### )

### (

### ( )

### )

### (

### ( )

### )

### (

### ( )

### )

### ( )

### (

### )

### (

### ( )

### )

### ( )

### (

### )

### (

### ( )

### )

### (

### ( )

### )

### (

### ( )

### )

### ( )

### (

### )

### (

### ( )

### )

0 0 0 0

2 _{2} 2

0 0

0 0 0 0

2 2

0 0

for 2 1, π

,

for 2 2,

*l* *l* *l* *l*

*l* *l*

*l*

*l* *l* *l* *l*

*l* *l*

*I* *u x* *K* *u x r* *K* *u x* *I* *u x r*

*l* *j*

*K* *u x* *I* *u x*

*r x*

*Y u x* *J* *u x r* *J* *u x Y u x r*

*l* *j*

*J* *u x* *Y u x*

−

= −

+

Ψ _{= }

−

_{=} _{−}

+

0, 0, 0

*I Y J* are the corresponding Bessel functions of order 0 and

### ( )

### (

0### )(

1### )

### (

### )(

1### )

### (

1### )(

### )

.*j* *j* *j* *k* *k*

*u* *x* = *x*−*a* *x*−*a* *x*−*a* − +*x* *a*_{+} − +*x* *a*_{−} − +*x* *a*

Proof. For the proof, we used the following properties of the Bessel functions (see [12][13][14])

### ( )

π π_{2}( )1 1 π

_{2}1

e e , π arg π ,

2 2

*i* *i*

*K*ν *z* *i* ν*H*ν *z* *z*

_{} _{}

= _{} _{} _{}− < ≤ _{}

_{ }

(10)
### ( )

π π_{2}( )2 1 π

_{2}1

e e , π arg π ,

2 2

*i* *i*

*K*_{ν} *z* *i* −ν*H*−_{ν} *z* − *z*

_{} _{}

= − _{} _{} _{}− < ≤ _{}

_{ }

(11)
### (

π### )

π### ( )

### (

### ) ( ) ( ) (

### )

e*im* e *im* πsin π csc π , ,

*K*ν *z* = − ν *K*ν *z* −*i* *m*ν ν *I*ν *z* *m*∈

### (12)

where ( )1

_{( )}

*H*ν *z* and *H*−( )ν2

### ( )

*z*are the Hankel’s functions of the order

### ν

.Consider the case when k is odd (for k even the proof is analogous). Also consider the region of Figure 1 then

*f t*

### ( )

is like Equation (3). Then 0*j*

γ →

### ∫

, 0*j*

γ ′ →

### ∫

for*j*=1, 2,,

*k*when →0, also

0 2π

*i*

γ = −

### ∫

when →0. Forthe integrals on Γ*j* and on Γ′*j* with *j*=1,,*k*+1 we obtain the following: if

*j* is of the form 2*j*−1, we used Equation (10) and Equation (11), then

### ( )

2 12 1 2 1 2 2 2 2

1

2 *j* e , d ,

*j* *j* *j*

*a* _{xt}

*j*
*a*

*i* *r x* *x*

*x*
−
− − −
−
−
′

Γ + Γ = − Ψ

### ∫

### ∫

### ∫

### (13) where

### ( )

### (

### ( )

### )

### (

### ( )

### )

### (

### ( )

### )

### (

### ( )

### )

### ( )

### (

### )

### (

### ( )

### )

0 2 2 0 2 2 0 2 2 0 2 2

2 2 2 2

0 2 2 0 2 2

, *j* *j* *j* *j* .

*j*

*j* *j*

*Y u* *x* *J* *u* *x r* *J* *u* *x Y u* *x r*

*r x*

*J* *u* *x* *Y u* *x*

− − − − − − − − Ψ = +

On the other hand, if *j* is of the form 2*j*, we used Equation (12) then

### ( )

2

2 2 2 1 2 1

1

2π *j* e , d ,

*j* *j* *j*

*a* _{xt}

*j*
*a*

*i* *r x* *x*

*x*

−

− − ′

Γ + Γ = − Ψ

### ∫

### ∫

### ∫

### (14) where

### ( )

### (

### ( )

### )

### (

### ( )

### )

### (

### ( )

### )

### (

### ( )

### )

### ( )

### (

### )

### (

### ( )

### )

0 2 1 0 2 1 0 2 1 0 2 1

2 1 2 _{2} 2

0 2 1 0 2 1

, .

π

*j* *j* *j* *j*

*j*

*j* *j*

*I* *u* *x* *K* *u* *x r* *K* *u* *x* *I* *u* *x r*

*r x*

*K* *u* *x* *I* *u* *x*

Note that

*u*

*j*

### ( )

*x*

is found using Equation (4) with *n*=2. As k is odd then

there are 1

2

*k*
+

integrals of the form Equation (13) and there are 2 1

*k*
+

of

the form Equation (14). Thus

### ( )

### ( )

0 1 1 1 1

2 1 2

2 2 2 1

1 1

2 2

2 2 2 1

1 1

1 1

2π 2 e , d 2π e , d .

*k* *k*

*j* *j*

*j* *j*

*k* *k*

*a* _{xt}*a* _{xt}

*j* *j*

*a* *a*

*j* *j*

*i* *i* *r x* *x* *i* *r x* *x*

*x* *x*

γ + +

−

− −

′ ′

Γ Γ Γ Γ

_{+} _{+}
− −
− −
= =
+ + + + +

= − − Ψ − Ψ

### ∫ ∫ ∫

### ∫

### ∫

### ∑

### ∫

### ∑

### ∫

(15)

On the other hand, for

1
*k*

γ +

### ∫

, we have that( ) 1

1 1 2 1

0 π cos

1 1 2 1

0

1 1 1 1

e d ,

1 1 1 1

*k*

*k* *k* *k*

*Rt*

*k* *k* *k*

*a* *a*

*a* *a*

*K* *r* *R*

*R* *R* *R* *R*

*a* *a*

*a* *a*

*K* *R*

*R* *R* *R* *R*

θ

γ+ α θ

+ −
+ −
_{} _{} _{} _{} _{} _{} _{}
_{} _{} + _{} + _{} _{}_{} + _{} + _{}
_{} _{} _{} _{} _{}_{} _{} _{}
<
_{} _{} _{} _{} _{} _{} _{}
_{} _{} + _{} + _{} _{}_{} + _{} + _{}
_{} _{} _{} _{} _{}_{} _{} _{}

### ∫

### ∫

(16) so, 1*k*γ+

### ∫

is 0 when*R*→ ∞ for

### δ

sufficiently small. Similarly to2
*k*

γ+

### ∫

### .

Thus, using0 2π

*i*

γ = −

### ∫

, Equation (15) and Equation (16), we arrive to### ( )

### ( )

### ( )

2 1 2 2 2 2 1 1 2 2 2 1 1 2 2 1 1 1 11 e , d

π

1

e , d

*j*
*j*
*j*
*j*
*k*
*a* _{xt}*j*
*a*
*j*
*k*
*a* _{xt}*j*
*a*
*j*

*f t* *r x* *x*

*x*

*r x* *x*

*x*
−
−
−
+
−
−
=
+
−
−
=

= + Ψ

+ Ψ

### ∑ ∫

### ∑ ∫

**3. Analytical Examples and Numerical Approximation **

In this section, we give some analytical examples corresponding to the exact solutions of the previous theorems and solve the integrals numerically for some particular cases.

Example 1. We consider that *k*=1, *n*=2, 0=*a*0<*a*1<*a*2= ∞ and

### ( )

### (

1### )

1

*F s* *s s* *a*

*s*

= + , then using (2)

### ( )

### (

### )

### (

### )

### (

### )

_{(}

_{)}

### (

### )

1 1 1 1 0 1 1 00 1 π

1 1

sin e d

π 2

1 1 π

1 1

sin e d

π 2

1 1

e d .

π

*a* _{xt}

*xt*
*a*

*a* _{xt}

*f t* *x* *x* *a* *x*

*x*

*x x* *a* *x*

*x*

*x* *x* *a* *x*

*x*
−
∞ _{−}
−
+
= _{} _{} − +
+
+ _{} _{} −
= − +

### ∫

### ∫

### ∫

Furthermore, since### (

### )

11 1 2 1 1

1 0 1

0

1

e d e π ,

2 2 2

*a t*

*a* _{xt}*a* *a t* *a t*

*x* *x* *a* *x* *I* *I*

*x*
−
− _{− +} _{=} _{+}
_{} _{} _{} _{}

### ∫

with *I*_{α} is the modified Bessel function of order

### α

, then### ( )

11 2 1 1

0 1

e .

2 2 2

*a t*

*a* *a t* *a t*

*f t* = − _{}*I* _{} _{}+*I* _{} _{}_{}

Example 2. We consider that *k*=2, *n*=2, 0=*a*0<*a*1<*a*2<*a*3= ∞ and

### ( )

### (

1### )(

2### )

1
*F s*

*s s* *a* *s* *a*

=

+ + , then using (7)

### ( )

### (

### )

### (

### )(

### )

### (

### )

### (

### )(

### )

### (

### )

### (

### )(

### )

1 2 1 2 0 1 2 1 2 1 20 1 π

1 1

sin e d

π 2

1 1 π

1 1

sin e d

π 2

2 1 π

1 1

sin e d ,

π 2
*a* _{xt}*a* _{xt}*a*
*xt*
*a*

*f t* *x*

*x* *x* *a* *x* *a*

*x*

*x x* *a* *x* *a*

*x*

*x x* *a* *x* *a*

−
−
∞ −
+
= _{} _{}
− + − +
+
+
− − +
+
+
− −

### ∫

### ∫

### ∫

so### ( )

### (

### )(

### )

### (

### )(

### )

1 2 0 1 2 1 2 1 1 e d π 1 1 e d π*a*

_{xt}*xt*

*a*

*f t* *x*

*x* *x* *a* *x* *a*

*x*
*x x* *a* *x* *a*
−
∞ −
=
− + − +
−
− −

### ∫

### ∫

Example 3. We consider that *k*=1, *n*=3, *m*=2, 0=*a*_{0}<*a*_{1}= <π *a*_{2}= ∞

and

### ( )

### (

### )

3 2 1 1*s*

*F s*

*s* _{s}* _{a}*
=

+ , then using (8)

### ( )

### (

### )

### ( )

### (

### )

### (

### )

_{( )}

2 1
2
2 2
2 1
1
2
2
0
1
2
2 1
0
1 π
1 π 1

sin e d

π 3 2

1 π 1 π

1 1

sin e d ,

π 3 2

*j*
*j*
*j*
*j*
*a* _{xt}*j*
*a*
*j*
*a* _{xt}*j*
*a*
*j*
*j* *j*

*f t* *u* *x* *x*

*x*

*j* *j*

*u* *x* *x*

*x*
+
+
+
−
=
−
+
=
+
= _{} − _{}
+ +
+ −

### ∑

### ∫

### ∑

### ∫

so### ( )

### ( )

_{(}

_{)}

3
π
0
3
π
1 6
1 1
1 π 1

sin e d

π 3 π

1 π 1

sin e d

π 6 π

3 5 6

1 2 , 7 6 , π , π

*xt*

*xt*
*s*

*f t* *x*

*x* *x*
*s*
*x*
*x* *x*
*t*
*F* *t*
−
∞ _{−}
= _{ }
− +
−
+ _{} _{}
−
Γ
= −

### ∫

### ∫

where 1*F*1 is the hypergeometric function.

Example 4. We consider that *r*>0, *k*=1, *a*_{1}=1

and

### ( )

### (

### (

### )

### )

### (

### )

### (

### )

0 0 1 1 1*k* *r s s*

*F s*

*s k* *s s*

+ =

+

then using (9)

### ( )

_{0}1 0

### ( )

1 1### ( )

1 1 1

1 e , d e , d

π

*xt* *xt*

*f t* *r x* *x* *r x* *x*

*x* *x*

∞

− −

= +

_{∫}

Ψ +_{∫}

Ψ

where

### ( )

### (

### ( )

### )

### (

### ( )

### )

### (

### ( )

### )

### (

### ( )

### )

### ( )

### (

### )

### (

### ( )

### )

0 0 0 0 0 0 0 0

0 2 2

0 0 0 0

, *Y u* *x* *J* *u* *x r* *J* *u* *x Y u* *x r* ,

*r x*

*J* *u* *x* *Y u* *x*

−

Ψ =

### ( )

### (

### ( )

### )

### (

### ( )

### )

### (

### ( )

### )

### (

### ( )

### )

### ( )

### (

### )

### (

### ( )

### )

0 1 0 1 0 1 0 1

1 2 _{2} 2

0 1 0 1

, *I* *u x* *K* *u x r* *K* *u x* *I* *u x r* ,

*r x*

*K* *u x* π *I* *u x*

−

Ψ =

+

### ( )

### (

### )

### ( )

### (

### )

0 1 and 1 1 .

*u* *x* = *x* − +*x* *u x* = *x x*−

On the other hand, we solve the integrals numerically corresponding to the exact solutions of Theorems 1, 2, 3 and 4 then we compare versus Stehfest numerical method. This method consist in numerically finding the Laplace inverse transform of the

*F s*

### ( )

using:### ( )

### ( )

1### ( )

### ( )

1

log 2 *n* log 2

*i*

*f t* *g i F* *i*

*t* = *t*

=

### ∑

where

### ( ) ( )

### ( )

### (

### ) (

### ) (

### )

1 1

1

min ,

2 2

2

1 1

2

2 !

1 .

! ! 1 ! ! 2 !

2

*n*
*n*

*i*
*n*

*i*

*i*
*k*

*k* *k*

*g i*

*n*

*k* *k k* *i* *k* *k* *i*

+

+
= _{} _{}
= −

_{−} _{−} _{−} _{−}

### ∑

[image:9.595.157.539.334.700.2]In Figure 2(a) and Figure 2(b) we observed in red color Equations ((2) and

Figure 2. Comparison of the exact solution (2), (7), (8), (9) with parameters *n*=2, *k*=5

### ,

*a*

_{1}=1, 2 π

*a* = , *a*_{3}=5, 2
4 e

Figure 3. Comparison of the exact solution (2) with paramteters *n*=2, *k*=5

### ,

*a*

_{1}=1, 2 π

*a* = , *a*_{3}=5, 2
4 e

*a* = , *a*_{5}=10 and Stehfest numerical method for different *n*_{1}, this

is for t small.

(7)) respectively with parameters *n*=2 , *k*=5 , *a*_{1}=1, *a*2=π, *a*3=5,
2

4

### e

*a*

### =

and*a*

_{5}=10

### .

In Figure 2(c) we observed Equation (8) in red color with the same previous parameters in conjunction with*m*=2

### .

In Figure 2(d) we observed Equation (9) in red color with the same previous parameters in conjunction with*r*=2. On the other hand, in all plots of Figure 2 Stehfest

numerical method is shown in black color with *n*1=14 and functions

*F s*

### ( )

corresponding to the aforetmentioned parameters for each cases. As expected in all cases the exact solution of the theorems proposed here coincide with Stehfest numerical method for large t values, however the stephens method does not approach well near the discontinuities of the function, for example in Figure 3

we see in red color the same exact solution of Figure 2(a) for small t values, and
in black color the Stehfest numerical method for different *n*1. In this figure is

shown that if *n*1 increase then Stehfest numerical method approximates better

to exact solution near zero but still the approximation is bad, this is because Stehfest numerical method does not give a good approximation near points where the functions are discontinuous (see [10]).

**4. Conclusion **

[image:10.595.253.494.65.296.2]
**References **

[1] Arciga, M.P., Ariza, F.J., Sanchez, J. and Salmeron, U. (2016) Fractional Stochastic Heat Equation on the Half-Line. Applied Mathematical Sciences, 10, 3095-3105. https://doi.org/10.12988/ams.2016.69243

[2] Di Paola, M., Pirrotta, A. and Valenza, A. (2011) Visco-Elastic Behavior through Fractional Calculus: An Easier Method for Best Fitting Experimental Results. Me-chanics of Materials, 43, 79-806. https://doi.org/10.1016/j.mechmat.2011.08.016 [3] Ekoue, F., Halloy A.F., Gigon, D., Plantamp, G. and Zajdman, E. (2013)

Max-well-Cattaneo Regularization of Heat Equation. International Scholarly and Scien-tific Research and Innovation, 7, 772-775.

[4] Golghanddashti, H. (2011) A New Analytically Derived Shape Factor for Gas-Oil Gravity Drainage Mechanism. Journal of Petroleum Science and Engineering, 77, 18-26. https://doi.org/10.1016/j.petrol.2011.02.004

[5] Huan, X., Hai,Q. and Xiao, J. (2013) Fractional Cattaneo Heat Equation in a Semi Infinite Medium. Chinese Physics B, 22, 014401-1-014401-6.

https://doi.org/10.1088/1674-1056/22/1/014401

[6] Shu, L. and Bing, C. (2016) Generalized Variational Principles for Heat Conduction Models Based on Laplace Transforms. International Journal of Heat and Mass Transfer, 103, 1176-1180. https://doi.org/10.1016/j.ijheatmasstransfer.2016.08.065 [7] Rehbinder, G. (1989) Darcyan Flow with Relaxation Effect. Applied Scientific

Re-search, 46, 45-72. https://doi.org/10.1007/BF00420002

[8] Hernndez, D., Nez, M. and Velasco, J. (2013) Telegraphic Double Porosity Models for Head Transient Behavior in Naturally Fractured Aquifers. Water Resources Re-search, 49, 4399-4408. https://doi.org/10.1002/wrcr.20347

[9] Wu, Y., Ehlig-Economides, C., Qin, G., Kang, Z., Zhang, W., Ajayi, B. and Tao, Q. (2007) A Triple-Continuum Pressure-Transient Model for a Naturally Fractured Vuggy Reservoir. Lawrence Berkeley National Laboratory, Berkeley, CA.

https://doi.org/10.2118/110044-MS

[10] Stehfest, H. (1970) Algorithm 368: Numerical Inversion of Laplace Transforms [d5].

Communications of the ACM, 13, 47-49. https://doi.org/10.1145/361953.361969 [11] Lfqvist, T. and Rehbinder, G. (1993) Transient Flow towards a Well in an Aquifer

including the Effect of Fluid Inertia. Applied Scientific Research, 51, 611-623. https://doi.org/10.1007/BF00868003

[12] Gradshteyn, I.S. and Ryzhik, I.M. (2014) Table of Integrals, Series, and Products. Academic Press, New York.

[13] Abramowitz, M. and Stegun, I.A. (1964) Handbook of Mathematical Functions: With Formulas, Graphs, and Mathematical Tables. Courier Corporation, New York. [14] Carslaw, H.S. and Jaeger, J.C. (1941) Operational Methods in Applied Mathematics,

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