# A family of algebraic-exponential formulas

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```					                           A Family
of
Algebraic–Exponential Integrals
╬

Francis J. O’Brien, Jr., Ph.D.
Aquidneck Indian Council
Newport, RI
Corrected Copy                                                                        March 26, 2013

One of the most important non–elementary transcendental (or special) functions in
mathematics is the gamma function, defined by the improper integral:


    t  1e  t dt ,   0
0

A number of alternative expressions for   can be derived by change of variable1.

A plot of   is shown in Figure 1. The domain is the entire number line, but most work for
real variables is restricted to   0.

1
Three transforms:
                  n
(a) if t  x ,    n  x n 1e  x dx,  0
n

0

         1  zx
(b) if t  zx,    z       x       e       dx,  0
0
1               1
 1
(c) if x  e ,      ln 
t
dx,   0
0
 x

 

Figure 1. The Gamma Function,  .
Source: Wolfram Mathematica, http://www.wolframalpha.com/.

Clearly,    0 for   0. Sokolnikoff (Chap. X, pp. 372 ff.) describes the theory of limits that
demonstrates the convergence of   for   0. The standard reference work on the gamma
function for real variables is the book by Emil Artin (1964). The References lists other works
that provide information on the gamma function.

——————

The author has created a small family of generalized indefinite and definite algebraic–
         
exponential real variable integrals of the form x  m exp  x  n based on the gamma function.
They are useful for scientific and engineering applications.
What follows is a summary of the primary core of gamma-based formulas that appear in
the Gradshteyn and Ryzhik (GR) integration handbook. A large number of species formulas can
be generated based on these and similar published integrals. Many such integrals along with
brief proofs appear in the book, 500 Integrals of Elementary and Special Functions (F. O’Brien,
2008). The complete list of solved formulas is given in the document “List of formulas in 500
Integrals      of       Elementary        and        Special       Functions”     (forthcoming),
http://www.docstoc.com/profile/waabu.

In the next section a formula number is given for integrals that appear in GR; otherwise it
is listed as “unpublished” with the section number in 500 Integrals based on the GR
classification of integrals.

NOTE: Some formulas in GR (7th Edition) contain misprints. For the errata sheet see,
http://www.mathtable.com/errata/gr7_errata.pdf

Algebraic–Exponential Integrals

Indefinite Integrals

Unpublished Formula, Section 2.3

m  ax                    1          1  t                m  1
 x e dx                         t
n

t  ax ,   n 
n
e dt
                                     
na

Unpublished Formula, Section 2.3

e  ax
n
1        z 1  t                     1 m
                            t
n
dx                    e dt       t  ax , z  n 
xm                na z                                       

NOTE: these functional forms provide a transformed gamma-like structure which can be used
for definite integral applications.

Formula 2.33.4
m  ax        n                x m 1 n  ax n   m  1  n m  n  ax n
 x e dx                           na
e       
na     x e dx

Formula 2.325.5
      n               n 
e  ax                    1  e  ax           e  ax
n

                dx                        na          dx 
xm                     m  1  x m 1        xmn 
                          

Formula 2.33.10
m  x
 x e dx  
n                 
  , x n   
n 

1               1  t                 m 1

n          t        e dt            
n
x n

NOTE: this formula and Formula 3.326.2 (listed below) are derived in the online paper at
n
docstoc.com, “  x m e  x dx and related integrals.” Related indefinite integral formulas are also
given there for the integrands:

n
    x m e ax           Unpublished Formula, Section 2.32
ax n
e
                       Formula 2.325.6
xm
n
e  x
                       Formula 2.33.9
xm

NOTE: Formulas are useful for generating log expansion formulas for integer values of the
 x ln x  dx given in Sect. 2.32 & Sect. 2.72 of 500 Integrals.
m      n
form,

Formula 2.33.11

x
m

exp  x n dx    
     1
exp  x n         
  1! x n
  k 1 
 
 k   k  1!  k 1 
                                         
n
 0                         
    m 1                      
  n  1,2,..., n  0,   0
                              

NOTE: only if the parameter  is an integer.

Formula 2.33.2

e
ax 2
dx 
1 
2 a
 
erfi a x ,

where erfi is the imaginary error function.

Formula 2.33.16

e
 x 2
dx 
1 
2 

erf  x ,         
where erf is the error function.

______________

Definite Integrals

NOTE: “Re” refers to real variables.

Unpublished Formula, Section 3.326

v
x
m

exp  x dx    n
              
  , u n    , v n           
m 1

u                                             n                              n
Re   0, Re n  0

NOTE: a general gamma function integral from which a number of integrals can be derived for
selected values of u & v by properties of the complete and lower/upper incomplete gamma
functions. Those well-known relations for  ,   , x  &  , x  inter alia are summarized in
O’Brien, pp. 6–9.

Formula 3.326.2

     

                  m 1
x           exp   x n dx                     
m

0                               n                     n
Re n  0, Re   0, Re                  0

NOTE: this formula is useful for many applications because of its simplicity.

Formula 3.381.8

 1  , u n 
u
m  x n                                                m 1
x      e         dx  n                              
n
0
[ u  0, Re   0, Re n  0, Re   0 ]

Formula 3.381.9

 1 , u n 

m  x n                                                  m 1
x     e          dx  n                               
n
u
[Re n  0, Re   0 ]

Formula 3.381.10

                       u              
m  x    m  x
n
m  x    n                 n
 x e dx  x e dx   x e dx 
0                       0               u

n v 1  , u n  n v 1 , u n              
m 1
n
[u  0, Re   0, Re n  0, Re   0 ]

Unpublished Formula, Section 4.215

                          1          1
m  x    n     1 1                               m 1
 x e dx  n    ln t 
      
dt        
n
0               0
Re n  0, Re   0, Re        0

NOTE: a general form of Euler’s 1730 original log formula for gamma function definition.

Unpublished Formula, Section 8.252

n  k 1
xm
n  k , x   n  k  1!e  x      
m 0      m!
[n  k  1  0, k  0,1,...]

NOTE: for integer values only, a generalization of n, x .

Formula 8.352.6

          n  k 1 x m 
 n  k , x   n  k  1! 1  e  x 
  m! 


             m0          
[n  k  1  0, k  0,1,...]

NOTE: for integer values only, a generalization of  n, x  .

Formula 8.352.8

 n  k , x  
 1nk 0, x   e  x nk 1  1m         m! 
n  k ! 


m 0                x m 1 

[n  k  1  0, k  0,1,...]

NOTE: for integer values only, a generalization of  n  1, x  .

✜
The author would like to acknowledge with deep gratitude the late Professor Alan Jeffrey for
extensive feedback in the creation of the formulas and assistance in their publication in
Gradshteyn and Ryzhik (6th & 7th Editions), Table of Integrals, Series, and Products.

References

NOTE: each reference provides information on the gamma function.

Abramowitz, Milton and Irene Stegun. Handbook of Mathematical Functions with Formulas,
Graphs, and Mathematical Tables. Washington, DC: U.S.Government Printing Office, 1964
(Reprinted Dover, New York, 1972).

Artin, Emil. The Gamma Function. New York, NY: Holt, Rinehart and Winston, 1964.

Carr, G.S. Formulas and Theorems in Pure Mathematics, 2nd ed. New York: Chelsea
Publishing Co., 1970.

Gradshteyn, I.S. and I.M. Ryzhik (7th Edition). Table of Integrals, Series, and Products. Alan
Jeffrey and Daniel Zwillinger, Editors. NY: Academic Press, 2007.
Errata: http://www.mathtable.com/errata/gr7_errata.pdf

Moll, Victor H. The integrals in Gradshteyn and Ryzhik. Part 4: The Gamma function.
arXiv:0705.0179v1 math.CA, 1 May 2007.

O’Brien, Francis J. Jr. 500 Integrals of Elementary and Special Functions, 2008.
ISBN: 1–4392-1981–8. http://www.docstoc.com/profile/waabu

m  x n
_______.     x    e         dx and related integrals, 2nd ed., Mar.16, 2013.
http://www.docstoc.com/profile/waabu

_______. List of formulas in 500 Integrals of Elementary and Special Functions. (2013,
forthcoming). http://www.docstoc.com/profile/waabu.

Sokolnikoff, I. S. Advanced Calculus. New York: McGraw Hill Book Co., 1939.

Whittaker, E. T. A Course of Modern Analysis: An Introduction to the General Theory of Infinite
Series and of Analytic Functions; With An Account of the Principal Transcendental Functions.
Cambridge University Press, 1902. Reprinted Whittaker, E. T. and G.N. Watson, 4th ed., 1934

Wolfram Research, Inc. The Wolfram Functions Site., 1998-2008.
http://functions.wolfram.com/GammaBetaErf/Factorial2/27/01/0002/