How can complex numbers be used to solve infinite product and sum equations?

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    2016
In summary, complex numbers are essential in solving infinite product and sum equations due to their ability to represent real and imaginary quantities and their useful properties. While they have limitations in some cases, they can be used to determine the convergence of infinite equations through various methods such as the ratio test.
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Euge
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Here is this week's POTW:

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Let $q$ be a complex number with $\lvert q \rvert < 1$. Show that

$$\prod_{n = 1}^\infty (1 - q^n) \sum_{n = -\infty}^\infty q^{n+2n^2} = \prod_{n = 1}^\infty (1 - q^{2n})^2$$

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Note: Do not worry about arguments of convergence.Remember to read the http://www.mathhelpboards.com/showthread.php?772-Problem-of-the-Week-%28POTW%29-Procedure-and-Guidelines to find out how to http://www.mathhelpboards.com/forms.php?do=form&fid=2!
 
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No one answered this week's problem. You can read my solution below.
Consider Jacobi's triple product identity

$$\sum_{n = -\infty}^\infty z^n q^{n^2} = \prod_{n = 1}^\infty (1 - q^{2n})(1 + zq^{2n-1})(1 + z^{-1}q^{2n-1})$$

Letting $q\mapsto q^2$ and setting $z = q$ in the identity yields

$$\sum_{n = -\infty}^\infty q^{n + 2n^2} = \prod_{n = 1}^\infty (1 - q^{4n})(1 + q^{4n-1})(1 + q^{4n-3})\tag{1}$$

Since

$$\prod_{n = 1}^\infty (1 - q^{4n-1})(1 + q^{4n-3}) = \prod_{n\; \text{even}} (1 - q^{2n-1}) \prod_{n\; \text{odd}} (1 - q^{2n-1}) = \prod_{n = 1}^\infty (1 - q^{4n-1})$$

then the product in $(1)$ becomes

$$\prod_{n = 1}^\infty (1 - q^{4n})(1 + q^{2n-1})\tag{2}$$

By factorization $1 - q^{4n} = (1 - q^{2n})(1 + q^{4n})$, the product in $(3)$ can be expressed

$$\prod_{n = 1}^\infty (1 - q^{2n})(1 + q^{2n})(1 + q^{2n-1})\tag{3}$$

The product in $(3)$ contains all products of even and odd parts of $1 + q^n$, so $(3)$ becomes

$$\prod_{n = 1}^\infty (1 - q^{2n})(1 + q^n)\tag{4}$$

Since $1 + q^n = \frac{1 - q^{2n}}{1 - q^n}$, $(4)$ can be written

$$\prod_{n = 1}^\infty \frac{(1 - q^{2n})^2}{1 - q^n}$$

Thus

$$\sum_{n = -\infty}^\infty q^{n + 2n^2} = \prod_{n = 1}^\infty \frac{(1 - q^{2n})^2}{1 - q^n}\tag{5}$$

Finally, multiplying both sides of $(5)$ by $\prod_{n = 1}^\infty (1 - q^n)$, the result is established.
 

FAQ: How can complex numbers be used to solve infinite product and sum equations?

How are complex numbers used to solve infinite product equations?

Complex numbers are used in infinite product equations because they can represent both real and imaginary quantities. This allows for the manipulation of complex expressions and the ability to find solutions to infinite products.

Can complex numbers be used to solve infinite sum equations?

Yes, complex numbers can be used to solve infinite sum equations. This is because complex numbers can be added and subtracted just like real numbers, making them useful in finding solutions to infinite sums.

How do complex numbers help in finding solutions to infinite product and sum equations?

Complex numbers have both real and imaginary components, which allows for the representation of complex expressions and the ability to find solutions to infinite product and sum equations. They also have properties such as commutativity and associativity, which make them useful in manipulating complex equations.

Are there any limitations to using complex numbers in solving infinite equations?

While complex numbers are useful in solving infinite equations, they do have limitations. For example, they cannot be used to find solutions to equations with infinite terms that do not follow a pattern. Additionally, some infinite equations may not have a finite solution, in which case complex numbers may not be useful.

How can one determine the convergence of an infinite product or sum using complex numbers?

To determine the convergence of an infinite product or sum, one can use the properties of complex numbers such as absolute convergence and ratio test. These properties can help determine if the infinite equation has a finite solution or if it diverges to infinity.

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