Complex sentence: The rings of quaternions and split-quaternions can both be represented by certain-matrices with complex entries, while there is no such matrix representation for octonions, since the multiplication is not associative in this case.
Split into simpler sentences: The rings of quaternions and split-quaternions can both be represented by certain-matrices with complex entries. There is no such matrix representation for octonions, since the multiplication operations is not associative in this case.