Ok so I have known about imaginary numbers for quite some time now. I also understand why we want them to exist (to have a solution for $x^2=-1$). I also remember reading that the complex numbers are closed under addition, multiplication and exponentiation. So my question is first of all: what are the quaternions and octonions (I remember seeing thinks like $j$ and $k$) and other Hypercomplex systems (as they are called). And second of all why did we create them. Also, I remember reading that the octonions are the largest of these hyperecomplex systems (meaning that any number in a hypercomplex system is also a number in the system of the octonions). Than you very much in advance.
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The place to look is John Baez's beautifully written article on The Octonions. The introduction is wonderfully entertaining and the relevant section you want to focus on is the Cayley-Dickson construction.