In algebra, a quartic function is a function of the form
where a is nonzero, which is defined by a polynomial of degree four, called a quartic polynomial.
A quartic equation, or equation of the fourth degree, is an equation that equates a quartic polynomial to zero, of the form
where a ≠ 0.^{[1]} The derivative of a quartic function is a cubic function.
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
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 or may not have 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, according to the Abel–Ruffini theorem.
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this solution, like all algebraic solutions of the quartic, requires the solution of a cubic to be found, it could not be published immediately.^{[2]} The solution of the quartic was published together with that of the cubic by Ferrari's mentor Gerolamo Cardano in the book Ars Magna.^{[3]}
The Soviet historian I. Y. Depman (ru) claimed that even earlier, in 1486, Spanish mathematician Valmes was burned at the stake for claiming to have solved the quartic equation.^{[4]} Inquisitor General Tomás de Torquemada allegedly told Valmes that it was the will of God that such a solution be inaccessible to human understanding.^{[5]} However, Petr 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.^{[6]} 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.^{[7]}
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.^{[8]}
Each coordinate of the intersection points of two conic sections is a solution 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 examples of other geometric problems whose solution involves 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.^{[9]}
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.^{[10]}
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.^{[11]}^{[12]}^{[13]}
Finding the distance of closest approach of two ellipses involves solving a quartic equation.
The eigenvalues of a 4×4 matrix are the roots of a quartic polynomial which is the characteristic polynomial of the matrix.
The characteristic equation of a fourthorder linear difference equation or differential equation is a quartic equation. An example arises in the TimoshenkoRayleigh theory of beam bending.^{[14]}
Intersections between spheres, cylinders, or other quadrics can be found using quartic equations.
Letting F and G be the distinct inflection points of the graph of a quartic function, 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:^{[15]}
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 a ≠ 0 the nature of its roots is mainly determined by the sign of its discriminant
This may be refined by considering the signs of four other polynomials:
such that P/8a^{2} is the second degree coefficient of the associated depressed quartic (see below);
such that R/8a^{3} is the first degree coefficient of the associated depressed quartic;
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:^{[16]}
There are some cases that do not seem to be covered, but in fact they cannot occur. For example, ∆_{0} > 0, P = 0 and D ≤ 0 is not one of the cases. In fact, if ∆_{0} > 0 and P = 0 then D > 0, since so this combination is not possible.
The four roots x_{1}, x_{2}, x_{3}, and x_{4} for the general quartic equation
with a ≠ 0 are given in the following formula, which is deduced from the one in the section on Ferrari's method by back changing the variables (see § 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
(if S = 0 or Q = 0, see § Special cases of the formula, below)
with
and
Consider the general quartic
It is reducible if Q(x) = R(x)×S(x), where R(x) and S(x) are nonconstant polynomials with rational coefficients (or more generally with coefficients in the same field as the coefficients of Q(x)). Such a factorization will take one of two forms:
or
In either case, the roots of Q(x) are the roots of the factors, which may be computed using the formulas for the roots of a quadratic function or cubic function.
Detecting the existence of such factorizations can be done using the resolvent cubic of Q(x). It turns out that:
In fact, several methods of solving quartic equations (Ferrari's method, Descartes' method, and, to a lesser extent, Euler's method) are based upon finding such factorizations.
If a_{3} = a_{1} = 0 then the biquadratic function
defines a biquadratic equation, which is easy to solve.
Let the auxiliary variable z = x^{2}. Then Q(x) becomes a quadratic q in z: q(z) = a_{4}z^{2} + a_{2}z + a_{0}. Let z_{+} and z_{−} be the roots of q(z). Then the roots of our quartic Q(x) are
The polynomial
is almost palindromic, as P(mx) = x^{4}/m^{2}P(m/x) (it is palindromic if m = 1). The change of variables z = x + m/x in P(x)/x^{2} = 0 produces the quadratic equation a_{0}z^{2} + a_{1}z + a_{2} − 2ma_{0} = 0. Since x^{2} − xz + m = 0, the quartic equation P(x) = 0 may be solved by applying the quadratic formula twice.
For solving purposes, 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 x^{4} + bx^{3} + cx^{2} + dx + e = 0, with b = a_{3}/a_{4}, c = a_{2}/a_{4}, d = a_{1}/a_{4}, and e = a_{0}/a_{4}. Substituting y − b/4 for x gives, after regrouping the terms, the equation y^{4} + py^{2} + qy + r = 0, where
If y_{0} is a root of this depressed quartic, then y_{0} − b/4 (that is y_{0} − a_{3}/4a_{4}) is a root of the original quartic and every root of the original quartic can be obtained by this process.
As explained in the preceding section, we may start with the 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) as
Then, we introduce a variable m into the factor on the lefthand side by adding 2y^{2}m + pm + m^{2} to both sides. After regrouping the coefficients of the power of y on the righthand side, this gives the equation

(1) 
which is equivalent to the original equation, whichever value is given to m.
As the value of m may be arbitrarily chosen, we will choose it in order to complete the square on the righthand side. This implies that the discriminant in y of this quadratic equation is zero, that is m is a root of the equation
which may be rewritten as

(1a) 
This is the resolvent cubic of the quartic equation. The value of m may thus be obtained from Cardano's formula. When m is a root of this equation, the righthand side of equation (1) is the square
However, this induces a division by zero if m = 0. This implies q = 0, 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 needs to choose a root of the cubic equation such that m ≠ 0. This is always possible except for the depressed equation y^{4} = 0.
Now, if m is a root of the cubic equation such that m ≠ 0, equation (1) becomes
This equation is of the form M^{2} = N^{2}, which can be rearranged as M^{2} − N^{2} = 0 or (M + N)(M − N) = 0. Therefore, equation (1) may be rewritten as
This equation is easily solved by applying to each factor the quadratic formula. Solving them we may write the four roots as
where ±_{1} and ±_{2} denote either + or −. As the two occurrences of ±_{1} must denote the same sign, this leaves four possibilities, one for each root.
Therefore, the solutions of the original quartic equation are
A comparison with the general formula above shows that √2m = 2S.
Descartes^{[18]} introduced in 1637 the method of finding the roots of a quartic polynomial by factoring it into two quadratic ones. Let
By equating coefficients, this results in the following system of equations:
This can be simplified by starting again with the depressed quartic y^{4} + py^{2} + qy + r, which can be obtained by substituting y − b/4 for x. Since the coefficient of y^{3} is 0, we get s = −u, and:
One can now eliminate both t and v by doing the following:
If we set U = u^{2}, then solving this equation becomes finding the roots of the resolvent cubic

(2) 
which is done elsewhere. This resolvent cubic is equivalent to the resolvent cubic given above (equation (1a)), as can be seen by substituting U = 2m.
If u is a square root of a nonzero root of this resolvent (such a nonzero 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 u for the square root of U merely exchanges the two quadratics with one another.
The above solution shows that a quartic polynomial with rational coefficients and a zero coefficient on the cubic term is factorable into quadratics with rational coefficients if and only if either the resolvent cubic (2) has a nonzero root which is the square of a rational, or p^{2} − 4r is the square of rational and q = 0; this can readily be checked using the rational root test.^{[19]}
A variant of the previous method is due to Euler.^{[20]}^{[21]} Unlike the previous methods, both of which use some root of the resolvent cubic, Euler's method uses all of them. Consider a depressed quartic x^{4} + px^{2} + qx + r. Observe that, if
then
Therefore, (r_{1} + r_{2})(r_{3} + r_{4}) = −s^{2}. In other words, −(r_{1} + r_{2})(r_{3} + r_{4}) is one of the roots of the resolvent cubic (2) and this suggests that the roots of that cubic are equal to −(r_{1} + r_{2})(r_{3} + r_{4}), −(r_{1} + r_{3})(r_{2} + r_{4}), and −(r_{1} + r_{4})(r_{2} + r_{3}). This is indeed true and it follows from Vieta's formulas. It also follows from Vieta's formulas, together with the fact that we are working with a depressed quartic, that r_{1} + r_{2} + r_{3} + r_{4} = 0. (Of course, this also follows from the fact that r_{1} + r_{2} + r_{3} + r_{4} = −s + s.) Therefore, if α, β, and γ are the roots of the resolvent cubic, then the numbers r_{1}, r_{2}, r_{3}, and r_{4} are such that
It is a consequence of the first two equations that r_{1} + r_{2} is a square root of α and that r_{3} + r_{4} is the other square root of α. For the same reason,
Therefore, the numbers r_{1}, r_{2}, r_{3}, and r_{4} are such that
the sign of the square roots will be dealt with below. The only solution of this system is:
Since, in general, there are two choices for each square root, it might look as if this provides 8 (= 2^{3}) choices for the set {r_{1}, r_{2}, r_{3}, r_{4}}, but, in fact, it provides no more than 2 such choices, because the consequence of replacing one of the square roots by the symmetric one is that the set {r_{1}, r_{2}, r_{3}, r_{4}} becomes the set {−r_{1}, −r_{2}, −r_{3}, −r_{4}}.
In order to determine the right sign of the square roots, one simply chooses some square root for each of the numbers α, β, and γ and uses them to compute the numbers r_{1}, r_{2}, r_{3}, and r_{4} from the previous equalities. Then, one computes the number √α√β√γ. Since α, β, and γ are the roots of (2), it is a consequence of Vieta's formulas that their product is equal to q^{2} and therefore that √α√β√γ = ±q. But a straightforward computation shows that
If this number is −q, then the choice of the square roots was a good one (again, by Vieta's formulas); otherwise, the roots of the polynomial will be −r_{1}, −r_{2}, −r_{3}, and −r_{4}, which are the numbers obtained if one of the square roots is replaced by the symmetric one (or, what amounts to the same thing, if each of the three square roots is replaced by the symmetric one).
This argument suggests another way of choosing the square roots:
Of course, this will make no sense if α or β is equal to 0, but 0 is a root of (2) only when q = 0, that is, only when we are dealing with a biquadratic equation, in which case there is a much simpler approach.
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 x^{4} + bx^{3} + cx^{2} + dx + e. 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} = −b/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 b = 0, this results in the polynomial

(3) 
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 about 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.
This gives exactly the same formula for the roots as the one provided by Descartes' method.
There is an alternative solution using algebraic geometry^{[22]} 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 x^{4} + px^{2} + qx + r = 0 may also be expressed as the x coordinates of the intersections of the two quadratic equations y^{2} + py + qx + r = 0 and y − x^{2} = 0 i.e., using the substitution y = x^{2} that two quadratics intersect in four points is an instance of Bézout's theorem. Explicitly, the four points are P_{i} ≔ (x_{i}, x_{i}^{2}) for the four roots x_{i} of the quartic.
These four points are not collinear because they lie on the irreducible quadratic y = x^{2} 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 λF_{1} + μF_{2} for any point [λ, μ] in the projective line — in other words, where λ and μ 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 = 6 different ways. Denote these Q_{1} = L_{12} + L_{34}, Q_{2} = L_{13} + L_{24}, and Q_{3} = L_{14} + L_{23}. Given any two of these, their intersection has exactly the four points.
The reducible quadratics, in turn, may be determined by expressing the quadratic form λF_{1} + μF_{2} 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 λ and μ and corresponds to the resolvent cubic.