Quartic
Solve Quartic equations (4th degree polynomials) in C and Haskell.
License: BSD.
(Also solves cubic, quadratic, and linear equations)
The two implementations are very close in their structure/design. Naturally, the Haskell implementation is shorter.
C implementation
There's one implementation for complex numbers, and another for real numbers (double), which only finds real roots.
Haskell implementation
This implementation is generic and should work for finite fields too (I think).
Note about Cubic equations
The solving of cubic equations only finds one root.
Finding all three roots for Complex Doubles isn't hard but I wanted to keep the code generic.
For that a type-class providing primitive roots of unity would be required.
The class providing the primitive square root of unity, (-1), exists in Haskell and it is Floating, but here we would need a primitive cubic root of unity..