Collection of Infinite Product and Series

Dr. Andreas Dieckmann, Physikalisches Institut der Uni Bonn

My interest in infinite products has its origin in the year 2000 in connection with the problem of the electrical field of a

line charge trapped inside a rectangular tube. After I learned that the double product can be solved using

elliptic theta functions I was hooked. The site has been growing ever since, and its focus has been expanded

to include Series as well.

These pages list thousands of expressions like products, sums, relations and limits shown in the following sections:

- Products involving Theta Functions

- q-Series

- special values of EllipticK and EllipticE

- Series of Hyperbolic Functions

- Series of Inverse Tangents ( Arcustangent )

- Series of Legendre Polynomials

- Series of Zeta PolyGamma PolyLog and related

- Series involving HarmonicNumber

- Series involving Hypergeometric Functions

- iterated expressions ( Tetration )

- some properties of ProductLog LerchPhi and PolyLog

{j, n, m} are Integer; {λ, q} > 0 and r are real; {z, , , , } may be complex; Γ[a] is Gamma[a];

some of the products possess pointlike poles, where the denominator of a factor gets zero for certain

values of z. The given domains may not be complete. Some of the expressions are well known,

others may be not; some were found in the depths of the world wide web, the first are derived

from the following product below.

→ any formula you decide to use should be numerically tested for validity in the users domain ←

Expressions communicated by other people are marked with (n) and are referenced below at the bottom.

Infinite Products : ( Back to Top )

This product converges and delivers infinite product representations for many functions if the {a, b, c, d} are

replaced by constants and simple functions in z :

Products of two Gammas :

Partial Fraction Decompositions :

General expression :

some special cases:

with n = 3 and m = 1 :

with n = 2 and m = 1 :

as simple function of k :

Special cases with m = 0 :

q - Product (0 < q < 1) :

Two kinds of decomposition of the same product :

With

(left hand side taken from the general expression for partial fractions above) equations like given for instance in the next line are valid for different functions f[k] :

If f[k] = and 0 < m then

More Products :

The maximum of the next function is found at | f )) = (-1/2 | ), its zeroes on the positive (negative) axis are the odd (even) Integers :

Euler’s product :

The next product approximates a Gauss function with :

Products of trig functions :

Products involving Theta Functions ( Back to Top )

is shorthand for EllipticTheta[n, z, q] and means EllipticThetaPrime[m, z, q].

( The above double product numerically converges best if k ≫ n. )

Series and Product Representations :

With [ 0 , q ] a few relations between the theta functions are

These limits give "needles" of height ±1 situated at the extreme values of Cos or Sin respectively (n∼1/(4λ)) :

Partial differential equation :

EllipticThetas with imaginary argument :

With z → 0 we get

Half Lambda :

Double Lambda :

Other relations :

Square and square root of q :

The following double products numerically converge best if k ≫ n.

Double product representation of the single theta functions :

If the product over k is carried out first we get products with Tanh and Coth :

The theta functions may be expressed through each other :

and exhibit a kind of double periodicity ({m, n} ∈ Integer) :

Trigonometric and hyperbolic Products :

With m = InverseEllipticNomeQ[Exp[-π λ]] and K[m] = EllipticK[m] :

q - Products :

In the following is ( 0 < q < 1 ) and [ 0 , q ] , ([ 0 , q ] =[ 0 , - q ] ) :

m = InverseEllipticNomeQ[q] and K[m] = EllipticK[InverseEllipticNomeQ[q]].

m = InverseEllipticNomeQ[q], K[m] = EllipticK[InverseEllipticNomeQ[q]] and E[m] = EllipticE[InverseEllipticNomeQ[q]]:

InverseEllipticNomeQ m[q], K[m[q]] and E[m[q]] expressed through infinite products or theta functions:

and can be expressed through *m**[**q**]* , K[m[q]] and E[m[q]] :

and similarly :

and :

and from combining the above like :

we get :

as q is getting larger than the branch cut of K and E is crossed, so the continuous and smooth complex functions are built from two parts :

or turned the other way round :

If the result of the imaginary transformation doesn't seem right, consider the following points :

• If in the resulting formula a sign change of the imaginary part as function of q occurs under a square root ( at q = Exp[- π / 2] ) then the square root may take the other sign

• Logs with complex arguments may end up on a wrong branch, try replacing Log[...] with Log[...] + n i 2 π

Theta Functions (z = ) expressed through EllipticK and m :

Series expansion of InverseEllipticNomeQ :

Series expansion of EllipticNomeQ :

Specific Values :

and are Ramanujans g functions, m = InverseEllipticNomeQ[] (for each n the even g and the odd G seem to show a somewhat simpler structure than their counterparts) :

products with q = :

m = InverseEllipticNomeQ[] :

special cases :

Theta Functions , specific values :

Other formulae and curiosities including sums of hyperbolic and inverse tangent (arctan) functions and q - series: ( Back to Top )

The following double sums numerically converge best if k ≫ n. For numerical checks the finite lower limit should be increased by 1 in case the running index is shifted by -1/2.

ϑ ‘’ represents the second derivative of ϑ (x,q) with respect to x, eg: means .

Series involving exponentials :

Theta functions as (alternating) series of shifted Gauss functions with same width :

Series involving :

Series of trig. functions :

The following approximations hold to about 2% over all a :

q - Series : ( Back to Top )

with → Cosh[ k Log[ q ]] + Sinh[ k Log[ q ]] the following expressions can be transformed into sums of hyperbolic functions.

The inner sum above gives the number of ascending sequences of length k in the permutations of n numbers.

For natural n PolyLog[-n, q] appears as a rational function in q.

There is a small stumble stone in the definition of LerchPhi in the neighbourhood of a = 0: .

( m = InverseEllipticNomeQ[q], K[m] = EllipticK[m], E[m] = EllipticE[m] ):

The appearing of n or n - 1 as summation stop index implies n ∈ Integer.

Lambert Type q Series:

The introduction of QPolyGamma[n, z, q] (nth derivative of the QDigamma function (z, q)) in Mathematica 7 allows expression of

other :

QFunction Identities :

With x ∈ Reals is

Real Part | Imaginary Part | |

n=1: | 0 | |

n=2: | ||

n=3: | ||

n=4: | ||

n=5: | ||

n=6: | ||

n=7: |

special values of EllipticK and EllipticE: ( Back to Top )

E[m] is EllipticE[m];

Series of Hyperbolic Functions: ( Back to Top )

ϑ ‘’ represents the second derivative of ϑ (x,q) with respect to x, eg: means .

For numerical checks the finite lower negative index limit should be increased by 1 where the running index is shifted by - 1/2.

m = InverseEllipticNomeQ[] :

The next expression uses an idea taken from “Weiss, J.D.(2014) The Summation of One Class of Infinite Series. Applied Mathematics, 5, 2815 - 2822. http://dx.doi.org/10.4236/am.2014.517269” :

both series approach - Log[2] from either below (Coth) or above (Tanh) for increasing z .

both series above approach z / (1 - z) from either below (Tanh) or above (Coth) for 1 < x.

both series above approach PolyLog[-j,z] from either below (Tanh) or above (Coth) for 1 < x.

m = InverseEllipticNomeQ[] :

m = InverseEllipticNomeQ[] :

Some hyperbolic Identities :

Series of CosIntegral: ( Back to Top )

Some Limits : ( Back to Top )

diverse Series : ( Back to Top )

The appearing of n or m as summation stop index implies n,m ∈ Integer.

Multiple Sums :

Borwein' s formula :

Benson' s formula:

more series :

Sum of the inverse k-gonal numbers :

Value of the series for the first k :

The sum gives following results for some rational s=p/q :

This sum alternates between ± π for z ∈ N :

In the following 4 expressions b = :

The next three expressions contain s = and t = :

Series of Logarithms : ( Back to Top )

(m = InverseEllipticNomeQ[q], K[m] = EllipticK[m], E[m] = EllipticE[m]), the appearing of n or n - 1 as summation stop index implies n ∈ Integer.

Next is the 'Fountain' function, plot it in the range of -50 < z < 10 with parametervalues of a between -3 and 1 !

Series of Inverse Tangents ( Arcustangent ) : ( Back to Top )

(m = InverseEllipticNomeQ[q], K[m] = EllipticK[m], E[m] = EllipticE[m]), the appearing of n or n - 1 as summation stop index implies n ∈ Integer.

LogGamma[z] is used, because it has a simpler branch strucure than Log[Gamma[z]] and avoids many discontinuities.

Some ArcTan Identities :

Series of Bessel Functions : ( Back to Top )

Series of Legendre Polynomials : ( Back to Top )

Series of Jacobi Polynomials :

Series of Hermite Polynomials :

Series of Zeta, PolyGamma, PolyLog and related : ( Back to Top )

Special Values of Zeta :

PolyGamma :

Take Gauss Multiplication formula of Gamma functions, apply D[Log[#], z] on both sides (#) m + 1 times gives

PolyLog and LerchPhi :

Special Values of PolyLog and LerchPhi :

Series of Beta Functions : ( Back to Top )

Recurrence relation : Beta[x, a + 1, b] + Beta[x, a, b + 1] = Beta[x, a, b];

Special values of InverseBetaRegularized :

Series of Gamma Functions : ( Back to Top )

Dougall' s Formula :

( K[x] = EllipticK[x], E[x] = EllipticE[x] ) :

note the offset of 1/2 that appears in the result of the second series above if a is set to 1.

Special values of InverseGammaRegularized :

Series involving HarmonicNumber : ( Back To Top )

The notation in the Exponent of the hypergeometric function says : take the derivative with respect to the seventh argument and then give it the value 1 - a or 1 + a.

Series involving Hypergeometric Functions : ( Back to Top )

A few Integrals : ( Back to Top )

Substitute and the Feynman - Hibbs Integral

Iterated Expressions ( Tetration ) : ( Back to Top )

The above function f[x] = - ProductLog[-Log[x]] / Log[x] has a special 'swapping' symmetry of basis and exponent in its argument:

f[x] is not defined beyond the maximum of its inverse function , namely < x, so with this symmetry it is plausible that the exponential tower

doesn't converge for x < as well, where it shows a kind of bifurcation.

Some Properties of ProductLog, LerchPhi and PolyLog ( Back to Top )

For 1/e ≤ x is ProductLog[ x Log[ x ]] = Log[ x ] .

For 0 ≤ x ≤ e is ProductLog[ - Log[ x ] / x] = - Log[ x ] .

For 0 ≤ x is Log[ ProductLog[ x ] ] = Log[ x ] - ProductLog[ x ] .

For purely imaginary arguments (x ∈ R) the complex decomposition of LerchPhi is :

These carry over with a = 0 to PolyLog :

The imaginary part of LerchPhi[x, s, a] with 1 ≤ x ∈ R is given by :

And with a = 0 follows the imaginary part of PolyLog[ s, x] :

The complex decomposition of with 1 ≤ x ∈ R and 0 ≤ {b, s} ∈ N into real and imaginary part can be obtained by the following expression :

explicitly for low s and b = 2 :

For all z ∈ C not on the real axis in ( -∞ < z < 1) and 0 ≤ {b, s} ∈ N the following inversion identity holds

(the If statement makes a '+' in case of an imaginary part of z larger than zero, a '-' in all other cases) :

The real part of with 1 ≤ x ∈ R is also given by

For (b ∈ N) is

The real and imaginary parts of LerchPhi[ , 2, 1/2 ] (on the unit circle) are

With Clausen type functions for LerchPhi defined as

(0 < s ∈ Integer, 0 ≤ θ ≤ 2π, the even CLi and the odd SLi are expressible through Euler Polynomials),

the real and imaginary parts of (on the unit circle) are

the expressions for with lowest s being

The above polynomials in a make nice approximations to trigonometric functions, getting better with increasing s.

The first non polynomial partnerfunctions are found to be

The function has an interesting derivative :

that means the lower CLi and SLi are essentially derivatives of the higher ones.

With the LerchPhi index n being a negative Integer the function appears as a rational function :

With the PolyLog index being a negative Integer the function appears as a rational function :

With Clausen type functions defined as

(0 < s ∈ Integer, 0 ≤ θ ≤ 2π, the even Ci and the odd Si are expressible through Bernoulli Polynomials),

the real and imaginary parts of (on the unit circle) are

the expressions for with lowest s being

The above polynomials in a make nice approximations to trigonometric functions, getting better with increasing s :

As before the derivative is with lowered index.

The first non polynomial partnerfunctions are found to be

The complex decomposition of PolyLog[s, x] with 1 ≤ x ∈ R and 0 ≤ s ∈ N can be obtained by the following expression :

explicitly for low s :

For all z ∈ C and not on the real axis in ( 0 ≤ z < 1 ) and 0 ≤ {b, s} ∈ N the following inversion identity holds :

LerchPhi and PolyLog display a similar (alternating with s) scheme in their real and imaginary parts :

The lowest Bernoulli and Euler Polynomials are

BernoulliB | EulerE | |

They are symmetric or antisymmetric (depending on n) with respect to x = 1/2 :

Connection to Bernoulli and Euler numbers :

Clausens Integral :

Contributors :

(1) Udo Ausserlechner, Infineon, per email

(2) Professor Don Zagier, MPI für Mathematik, Bonn

(3) Stephen, per email