A First Course in String Theory
Recall that a group is a set which is closed under an associative multiplication; it contains an identity element, and each element has a multiplicative inverse. Verify that and , as described above, are groups.
A group is a set, G, together with an operation (called the group law of G) that combines any two elements a and b to form another element, denoted or . To qualify as a group, the set and operation, , must satisfy four requirements known as the group axioms:
For all a, b in G, the result of the operation, , is also in G.
For all a, b and c in G, .
There exists an element e in G such that, for every element a in G, the equation holds. Such an element is unique, and thus one speaks of the identity element.
For each a in G, there exists an element b in G, commonly denoted (or , if the operation is denoted “+”), such that , where e is the identity element.
— Wikipedia on Group (mathematics)
The axioms for a group are short and natural… Yet somehow hidden behind these axioms is the monster simple group, a huge and extraordinary mathematical object, which appears to rely on numerous bizarre coincidences to exist. The axioms for groups give no obvious hint that anything like this exists.
— Richard Borcherds in Mathematicians: An Outer View of the Inner World
2019.07.28 Sunday ACHK