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In mathematics, a quartic function, is a function of the form
where a is nonzero, which is defined by a polynomial of degree four, called quartic polynomial.
Sometimes the term biquadratic is used instead of quartic, but, usually, biquadratic function refers to a quadratic function of a square (or, equivalently, to the function defined by a quartic polynomial without terms of odd degree), having the form
A quartic equation, or equation of the fourth degree, is an equation consisting in equating to zero a quartic polynomial, of the form
where a ≠ 0.
The derivative of a quartic function is a cubic function.
Since a quartic function is defined by a polynomial of even degree, it has the same infinite limit when the argument goes to positive or negative infinity. If a is positive, then the function increases to positive infinity at both ends; and thus the function has a global minimum. Likewise, if a is negative, it decreases to negative infinity and has a global maximum. In both cases it may have, but not always, another local maximum and another local minimum.
The degree four (quartic case) is the highest degree such that every polynomial equation can be solved by radicals.
Lodovico Ferrari is credited with the discovery of the solution to the quartic in 1540, but since this solution, like all algebraic solutions of the quartic, requires the solution of a cubic to be found, it could not be published immediately.^{[1]} The solution of the quartic was published together with that of the cubic by Ferrari's mentor Gerolamo Cardano in the book Ars Magna (1545).
The Soviet historian I. Y. Depman claimed that even earlier, in 1486, Spanish mathematician Valmes was burned at the stake for claiming to have solved the quartic equation.^{[2]} Inquisitor General Tomás de Torquemada allegedly told Valmes that it was the will of God that such a solution be inaccessible to human understanding.^{[3]} However Beckmann, who popularized this story of Depman in the west, said that it was unreliable and hinted that it may have been invented as Soviet antireligious propaganda.^{[4]} Beckmann's version of this story has been widely copied in several books and internet sites, usually without his reservations and sometimes with fanciful embellishments. Several attempts to find corroborating evidence for this story, or even for the existence of Valmes, have failed.^{[5]}
The proof that four is the highest degree of a general polynomial for which such solutions can be found was first given in the Abel–Ruffini theorem in 1824, proving that all attempts at solving the higher order polynomials would be futile. The notes left by Évariste Galois prior to dying in a duel in 1832 later led to an elegant complete theory of the roots of polynomials, of which this theorem was one result.^{[6]}
The Cassini oval, which is the locus of points all of which have the same product of distances to a pair of foci, is a quartic in two variables.
The Cartesian oval, which is the locus of points all of which have the same weighted sum of distances to two foci, is a quartic in two variables.
Limaçons are quartics in two variables.
Each coordinate of the intersection points of two conic sections is the solutions of a quartic equation. The same is true for the intersection of a line and a torus. It follows that quartic equations often arise in computational geometry and all related fields such as computer graphics, computeraided design, computeraided manufacturing and optics. Here are example of other geometric problems whose solution amounts of solving a quartic equation.
In computeraided manufacturing, the torus is a shape that is commonly associated with the endmill cutter. To calculate its location relative to a triangulated surface, the position of a horizontal torus on the Zaxis must be found where it is tangent to a fixed line, and this requires the solution of a general quartic equation to be calculated.
A quartic equation arises also in the process of solving the crossed ladders problem, in which the lengths of two crossed ladders, each based against one wall and leaning against another, are given along with the height at which they cross, and the distance between the walls is to be found.
In optics, Alhazen's problem is "Given a light source and a spherical mirror, find the point on the mirror where the light will be reflected to the eye of an observer." This leads to a quartic equation.^{[7]}^{[8]}^{[9]}
Finding the distance of closest approach of two ellipses involves solving a quartic equation.
Letting F and G be the distinct inflection points of a quartic, and letting H be the intersection of the inflection secant line FG and the quartic, nearer to G than to F, then G divides FH into the golden section:^{[10]}
Moreover, the area of the region between the secant line and the quartic below the secant line equals the area of the region between the secant line and the quartic above the secant line. One of those regions is disjointed into subregions of equal area.
Given the general quartic equation
with real coefficients and the nature of its roots is mainly determined by the sign of its discriminant
This may be refined by considering the signs of three other polynomials:
such that is the second degree coefficient of the associated depressed quartic (see below);
which is 0 if the quartic has a triple root; and
which is 0 if the quartic has two double roots.
The possible cases for the nature of the roots are as follows:^{[11]}
There are some cases that do not seem to be covered, but they can not occur. For example > 0, P = 0 and D ≤ 0 is not one of the cases. However if > 0 and P = 0 then D > 0 so this combination is not possible.
The four roots for the general quartic equation
with a ≠ 0 are given in the following formula, which is deduced from the one in the section Solving by factoring into quadratics by back changing the variables (see section Converting to a depressed quartic) and using the formulas for the quadratic and cubic equations.
where p and q are the coefficients of the second and of the first degree respectively in the associated depressed quartic
and where
with
and
If the value of is a nonreal complex number. In this case, either all roots are nonreal or they are all real. In the latter case, the value of is also real, and one may prefer to express it in a purely real way, by using trigonometric functions, as follows:
where
If and the sign of has to be chosen to have that is one should define as maintaining the sign of
If then one must change the choice of the cubic root in for having This is always possible except if the quartic may be factored into The result is then correct, but misleading hiding the fact that no cubic root is needed in this case. In fact this case may occur only if the numerator of is zero, and the associated depressed quartic is biquadratic; it may thus be solved by the method described below.
If and and thus also at least three roots are equal, and the roots are rational functions of the coefficients.
If and the above expression for the roots is correct but misleading, hiding the fact that the polynomial is reducible and no cubic root is needed to represent the roots.
Consider the general quartic
It is reducible if Q=RS, where R and S are nonconstant polynomials with rational coefficients (or more generally with coefficients in the same field as the coefficients of Q). There are two ways to write such a factorization: Either
or
In either case, the roots of Q are the roots of the factors, which may be computed by solving quadratic or cubic equations.
Detecting such factorizations can be done by using the factor function of every computer algebra system. But, in many cases, it may be done by handwritten computation. In the preceding section, we have already seen that the polynomial is always reducible if its discriminant is zero (this is true for polynomials of every degree).
A very special case of the first case of factorization is when a_{0}=0. This implies that x_{1}=0 is a first root, b_{3}=a_{4}, b_{2}=a_{3}, b_{1}=a_{2}, b_{0}=a_{1}, and the other roots may be computed by solving a cubic equation.
If then and we have a factorization of the first kind with x_{1}=1. Similarly, if then and we have a factorization of the first kind with x_{1}=1.
Once a root x_{1} is known, the second factor of the factorization of the first kind is the quotient of the Euclidean division of Q by xx_{1}. It is
If are small integers a factorization of the first kind is easy to detect: if with p and q coprime integers, then q divides evenly a_{4}, and p divides evenly a_{0}. Thus, computing for every possible values of p and q allows to find the rational roots, if any.
In the case of two quadratic factors or of large integer coefficients, the factorization is harder to compute, and, in general, it is better to use the factor function of a computer algebra system (see polynomial factorization for a description of the algorithms that are involved).
If then the biquadratic function
defines a biquadratic equation, which is easy to solve.
Let Then Q becomes a quadratic q in
Let and be the roots of q. Then the roots of our quartic Q are
The polynomial
is almost palindromic, as
(it is palindromic if m = 1).
The change of variables in produces the quadratic equation As x^{2}  xz + m= 0, the quartic equation
may be solved by applying twice the quadratic formula.
For solving purpose, it is generally better to convert the quartic into a depressed quartic by the following simple change of variable. All formulas are simpler and some methods work only in this case. The roots of the original quartic are easily recovered from that of the depressed quartic by the reverse change of variable.
Let
be the general quartic equation we want to solve.
Dividing by a_{4}, provides the equivalent equation
with
Substituting x by gives, after a simple term regrouping, the equation
where
If y_{1}, y_{2}, y_{3}, y_{4} are the roots of this depressed quartic, then the roots of the original quartic are
As explained in the preceding section, we may start with a depressed quartic equation
This depressed quartic can be solved by means of a method discovered by Lodovico Ferrari. The depressed equation may be rewritten (this is easily verified by expanding the square and regrouping all terms in the lefthand side)
Then, we introduce a variable y into the factor on the lefthand side by adding to both sides. After regrouping the coefficients of the power of u in the righthand side, this gives the equation
which is equivalent to the original equation, whichever value is given to y.
As the value of y may be arbitrarily chosen, we will choose it in order to get a perfect square in the righthand side. This implies that the discriminant in u of this quadratic equation is zero, that is y is a root of the equation
which may be rewritten
The value of y may thus be obtained from the formulas provided in the article Cubic equation.
When y is a root of equation (4), the righthand side of equation (3) the square of
However, this induces a division by zero if This implies and thus that the depressed equation is biquadratic, and may be solved by an easier method (see above). This was not a problem at the time of Ferrari, when one solved only explicitly given equations with numeric coefficients. For a general formula that is always true, one thus need to choose a root of the cubic equation such that This is always possible unless for the depressed equation x^{4}=0.
Now, if y is a root of the cubic equation such that equation (3) may be rewritten
and the equation is easily solved by applying to each factor the formula for quadratic equations. Solving them we may write the four roots as
where and denote either + or . As the two occurrences of must denote the same sign, this leave four possibilities, one for each root.
Therefore the solutions of the original quartic equation are
One can solve a quartic by factoring it into a product of two quadratics.^{[14]} Let
By equating coefficients, this results in the following set of simultaneous equations:
This can be simplified by starting again with a depressed quartic where , which can be obtained by substituting for , then , and:
One can now eliminate both and by doing the following:
If we set , then this equation turns into the resolvent cubic equation
which is solved elsewhere. Then, if p is a square root of a nonzero root of this resolvent (such a non zero root exists except for the quartic x^{4}, which is trivially factored),
The symmetries in this solution are as follows. There are three roots of the cubic, corresponding to the three ways that a quartic can be factored into two quadratics, and choosing positive or negative values of for the square root of merely exchanges the two quadratics with one another.
The above solution shows that the quartic polynomial with a zero coefficient on the cubic term is factorable into quadratics with rational coefficients if and only if either the resolvent cubic has a nonzero root which is the square of a rational, or is the square of rational and c=0; this can readily be checked using the rational root test.
The symmetric group S_{4} on four elements has the Klein fourgroup as a normal subgroup. This suggests using a resolvent cubic whose roots may be variously described as a discrete Fourier transform or a Hadamard matrix transform of the roots; see Lagrange resolvents for the general method. Denote by x_{i}, for i from 0 to 3, the four roots of
If we set
then since the transformation is an involution we may express the roots in terms of the four s_{i} in exactly the same way. Since we know the value s_{0} = a/2, we only need the values for s_{1}, s_{2} and s_{3}. These are the roots of the polynomial
Substituting the s_{i} by their values in term of the x_{i}, this polynomial may be expanded in a polynomial in s whose coefficients are symmetric polynomials in the x_{i}. By the fundamental theorem of symmetric polynomials, these coefficients may be expressed as polynomials in the coefficients of the monic quartic. If, for simplification, we suppose that the quartic is depressed, that is a=0, this results in the polynomial
This polynomial is of degree six, but only of degree three in s^{2}, and so the corresponding equation is solvable by the method described in the article Cubic function. By substituting the roots in the expression of the x_{i} in terms of the s_{i}, we obtain expression for the roots. In fact we obtain, apparently, several expressions, depending on the numbering of the roots of the cubic polynomial and of the signs given to their square roots. All these different expressions may be deduced from one of them by simply changing the numbering of the x_{i}.
These expressions are unnecessarily complicated, involving the cubic roots of unity, which can be avoided as follows. If s is any nonzero root of (3), and if we set
then
We therefore can solve the quartic by solving for s and then solving for the roots of the two factors using the quadratic formula.
Note that this gives exactly the same formula for the roots as the preceding section.
An alternative solution using algebraic geometry is given in (Faucette 1996), and proceeds as follows (more detailed discussion in reference). In brief, one interprets the roots as the intersection of two quadratic curves, then finds the three reducible quadratic curves (pairs of lines) that pass through these points (this corresponds to the resolvent cubic, the pairs of lines being the Lagrange resolvents), and then use these linear equations to solve the quadratic.
The four roots of the depressed quartic may also be expressed as the x coordinates of the intersections of the two quadratic equations i.e., using the substitution that two quadratics intersect in four points is an instance of Bézout's theorem. Explicitly, the four points are for the four roots of the quartic.
These four points are not collinear because they lie on the irreducible quadratic and thus there is a 1parameter family of quadratics (a pencil of curves) passing through these points. Writing the projectivization of the two quadratics as quadratic forms in three variables:
the pencil is given by the forms for any point in the projective line – in other words, where and are not both zero, and multiplying a quadratic form by a constant does not change its quadratic curve of zeros.
This pencil contains three reducible quadratics, each corresponding to a pair of lines, each passing through two of the four points, which can be done different ways. Denote these Given any two of these, their intersection is exactly the four points.
The reducible quadratics, in turn, may be determined by expressing the quadratic form as a 3×3 matrix: reducible quadratics correspond to this matrix being singular, which is equivalent to its determinant being zero, and the determinant is a homogeneous degree three polynomial in and and corresponds to the resolvent cubic.
