he discovered it in 1823

The galois theory is used to solve radicals.

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1832 is when he wrote about the groupl theory.

Stephen U. Chase has written:

'Hopf algebras and Galois theory' -- subject(s): Galois theory, Hopf algebras

Many consider him the father of modern algebra.

Galois theory was named after him and is very important for many reasons one of which is it provides a connection between field theory in modern algebra and group theory.