| MacMahon then worked on invariants of binary quartic forms, following Cayley and Sylvester. |
| Cayley also put a third question, a different and deeper question about irreducible invariants of a binary quantic. |
| The author examines solvability in quantum mechanics and classically superfluous invariants. |
| He distinguished between these structures using numerical invariants based on the Todd polynomials. |
| One of his most-engrossing studies concerned the relationships between the theory of invariants and number theory. |
| Over the years, mathematicians have developed a variety of such invariants, often expressed in the form of a polynomial algebraic expression. |
