János Pintz

In mathematics, the indefinite product operator is the inverse operator of $Q(f(x))={\frac {f(x+1)}{f(x)}}$. It is like a discrete version of the indefinite product integral. Some authors use term discrete multiplicative integration

Thus

$Q(\prod _{x}f(x))=f(x)\,.$

More explicitly, if $\prod _{x}f(x)=F(x)\,$, then

${\frac {F(x+1)}{F(x)}}=f(x)\,.$

If F(x) is a solution of this functional equation for a given f(x), then so is CF(x) for any constant C. Therefore each indefinite product actually represents a family of functions, differing by a multiplicative constant.

Period rule

If $T\,$ is a period of function $f(x)\,$ then

$\prod _{x}f(Tx)=Cf(Tx)^{x-1}\,$

Connection to indefinite sum

Indefinite product can be expressed in terms of indefinite sum:

$\prod _{x}f(x)=\exp \left(\sum _{x}\ln f(x)\right)\,$

Alternative usage

Some authors use the phrase "indefinite product" in a slightly different but related way to describe a product in which the numerical value of the upper limit is not given. e.g.

$\prod _{k=1}^{n}f(k)$.

Rules

$\prod _{x}f(x)g(x)=\prod _{x}f(x)\prod _{x}g(x)\,$

$\prod _{x}f(x)^{a}=\left(\prod _{x}f(x)\right)^{a}\,$

$\prod _{x}a^{f(x)}=a^{\sum _{x}f(x)}\,$

List of indefinite products

This is a list of indefinite products $\prod _{x}f(x)\,$. Not all functions have an indefinite product which can be expressed in elementary functions.

$\prod _{x}a=Ca^{x}\,$

$\prod _{x}x=C\,\Gamma (x)\,$

$\prod _{x}{\frac {x+1}{x}}=Cx$

$\prod _{x}{\frac {x+a}{x}}={\frac {C\,\Gamma (x+a)}{\Gamma (x)}}$

$\prod _{x}x^{a}=C\,\Gamma (x)^{a}\,$

$\prod _{x}ax=Ca^{x}\Gamma (x)\,$

$\prod _{x}a^{x}=Ca^{{\frac {x}{2}}(x-1)}\,$

$\prod _{x}a^{\frac {1}{x}}=Ca^{\frac {\Gamma '(x)}{\Gamma (x)}}\,$

$\prod _{x}x^{x}=C\,e^{\zeta ^{\prime }(-1,x)-\zeta ^{\prime }(-1)}=C\,e^{\psi ^{(-2)}(z)+{\frac {z^{2}-z}{2}}-{\frac {z}{2}}\ln(2\pi )}=C\,\operatorname {K} (x)\,$

(see K-function)

$\prod _{x}\Gamma (x)={\frac {C\,\Gamma (x)^{x-1}}{\operatorname {K} (x)}}=C\,\Gamma (x)^{x-1}e^{{\frac {z}{2}}\ln(2\pi )-{\frac {z^{2}-z}{2}}-\psi ^{(-2)}(z)}=C\,\operatorname {G} (x)\,$

(see Barnes G-function)

$\prod _{x}\operatorname {sexp} _{a}(x)={\frac {C\,(\operatorname {sexp} _{a}(x))'}{\operatorname {sexp} _{a}(x)(\ln a)^{x}}}\,$

(see super-exponential function)

$\prod _{x}x+a=C\,\Gamma (x+a)\,$

$\prod _{x}ax+b=C\,a^{x}\Gamma \left(x+{\frac {b}{a}}\right)\,$

$\prod _{x}ax^{2}+bx=C\,a^{x}\Gamma (x)\Gamma \left(x+{\frac {b}{a}}\right)\,$

$\prod _{x}x^{2}+1=C\,\Gamma (x-i)\Gamma (x+i)$

$\prod _{x}x+{\frac {1}{x}}={\frac {C\,\Gamma (x-i)\Gamma (x+i)}{\Gamma (x)}}$

$\prod _{x}\csc x\sin(x+1)=C\sin x\,$

$\prod _{x}\sec x\cos(x+1)=C\cos x\,$

$\prod _{x}\cot x\tan(x+1)=C\tan x\,$

$\prod _{x}\tan x\cot(x+1)=C\cot x\,$

See also

• Indefinite sum
• Product integral
• List of derivatives and integrals in alternative calculi

References

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Further reading

• http://reference.wolfram.com/mathematica/ref/Product.html -Indefinite products with Mathematica
• http://www.math.rwth-aachen.de/MapleAnswers/660.html - bug in Maple V to Maple 8 handling of indefinite product
• Markus Müller. How to Add a Non-Integer Number of Terms, and How to Produce Unusual Infinite Summations
• Markus Mueller, Dierk Schleicher. Fractional Sums and Euler-like Identities