I can get my head around this so someone explain it please.
$(1)$ From Galois theory it is known there is no formula to solve a general quintic equation.
But it is known a general quintic can be solved for the 5 roots exactly. Back in 1858 Hermite and Kronecker independently showed the quintic can be exactly solved for (using elliptic modular function). Also I think they're maybe other solution for the quintic which means a formula for each of the 5 roots.
So why is the claim in Galois theory that there is no formula to solve it? I know I am missing something here because the above $(1)$ is an established result.
So what is the value in saying, using Galois theory we do not have a formula for the 5 roots? Since for practical purposes we can actually find the 5 roots each time using say for example the formula based on elliptic modular functions.
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$\begingroup$Galois-theory only says that there is no general formula to solve a quintic equation in terms of radicals. That is, there is no formula only using the arithmetic operations "sum, multiplication etc. and taking the $n$-th root".
For instance for the polynomial $x^5 - 4x + 2$ it is known that it has a root that is not expressible in the above mentioned operations (as its Galois-group is $S_5$). (Edit: Another example is $x^5 + x + 1$ which also has Galois-group $S_5$. If you Wolframalpha this polynomial you see nicely how four of its roots can be expressed by radicals, but the fifth can't.)
The solution you mean is the solution using Bring radicals - wikipedia-article here: -, which is not a contradiction, as it is not expressed in form of radicals (in the sense of $n$-th roots of something).
$\endgroup$ 4 $\begingroup$The problem of solving algebraic equations algebraically is one of the oldest problems humans have considered —thousands of years old— arising from everything, going from the subdivision of inheritance to riddles by mithological figures to the construction of actual buildings. The fact that we know exactly when we can do it and, when it is possible, that we can in fact carry out the construction of solutions is one of the greatest achievements of mankind.
That's the value of it.
No one cares about infinite precision.
$\endgroup$ 7 $\begingroup$I'd like to share that the principal quintic $x^5+ax^2+bx+c=0$ is algebraically solvable by the "Fórmula Luderiana para Equação Quíntica" (Luderian Formula for Quintic Equation) just check it out at slideshare for details.
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