/kloc-in the 6th century, Italian mathematicians Tattaglia, Cardan and others discovered the formula for finding the root of a cubic equation with one variable, and Ferrari discovered the formula for finding the root of a quartic equation. At that time, mathematicians were optimistic that they would soon be able to write the root formulas of quintic equations, cubic equations and even higher-order equations. However, hundreds of years have passed, and no one can find such a formula for finding the root.

About 300 years later, in 1825, a Norwegian scholar Abel finally proved a general algebraic equation. If the degree of the equation n≥5, it cannot be solved by roots. That is, there is no formula for finding the root of the general quintic equation. This is the famous Abel theorem.