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Casus irreducibilis (from Latin 'the irreducible case') is the name given by mathematicians of the 16th century to cubic equations that cannot be solved in terms of real radicals, that is to those equations such that the computation of the solutions cannot be reduced to the computation of square and cube roots.
The solutions of the cubic equation do not necessarily belong to the same field as the coefficients. For example, some cubic equations with rational coefficients have roots that are irrational (and even non-real) complex numbers.
The non-real factors come in pairs which when multiplied give quadratic polynomials with real coefficients. Since every polynomial with complex coefficients can be factored into 1st-degree factors (that is one way of stating the fundamental theorem of algebra ), it follows that every polynomial with real coefficients can be factored into ...
But clearly not all real numbers are solutions to the original equation. The problem is that multiplication by zero is not invertible : if we multiply by any nonzero value, we can reverse the step by dividing by the same value, but division by zero is not defined, so multiplication by zero cannot be reversed.
Complex numbers allow solutions to all polynomial equations, even those that have no solutions in real numbers. More precisely, the fundamental theorem of algebra asserts that every non-constant polynomial equation with real or complex coefficients has a solution which is a complex number.
If only real numbers are considered, = is not a solution, as it leads to a non-real subexpression in the given equation. This requires the original equation to consist of integer -coefficient linear combinations of logarithms w.r.t. a unique base, and the logarithm arguments to be polynomials in x .
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Single Multiplicity-2 (SM2): when the general quartic equation can be expressed as () () =, where , , and are three different real numbers or is a real number and and are a couple of non-real complex conjugate numbers. This case is divided into two subcases, those that can be reduced to a biquadratic equation and those in which this is impossible.