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Concept
Infinite product
Summary
In mathematics, for a sequence of complex numbers a1, a2, a3, ... the infinite product
is defined to be the limit of the partial products a1a2...an as n increases without bound. The product is said to converge when the limit exists and is not zero. Otherwise the product is said to diverge. A limit of zero is treated specially in order to obtain results analogous to those for infinite sums. Some sources allow convergence to 0 if there are only a finite number of zero factors and the product of the non-zero factors is non-zero, but for simplicity we will not allow that here. If the product converges, then the limit of the sequence an as n increases without bound must be 1, while the converse is in general not true.
The best known examples of infinite products are probably some of the formulae for π, such as the following two products, respectively by Viète (Viète's formula, the first published infinite product in mathematics) and John Wallis (Wallis product):
The product of positive real numbers
converges to a nonzero real number if and only if the sum
converges. This allows the translation of convergence criteria for infinite sums into convergence criteria for infinite products. The same criterion applies to products of arbitrary complex numbers (including negative reals) if the logarithm is understood as a fixed branch of logarithm which satisfies ln(1) = 0, with the proviso that the infinite product diverges when infinitely many an fall outside the domain of ln, whereas finitely many such an can be ignored in the sum.
For products of reals in which each , written as, for instance, , where , the bounds
show that the infinite product converges if the infinite sum of the pn converges. This relies on the Monotone convergence theorem. We can show the converse by observing that, if , then
and by the limit comparison test it follows that the two series
are equivalent meaning that either they both converge or they both diverge.
The same proof also shows that if for some , then converges to a non-zero number if and only if converges.
Official source
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