Setting g(x) = 0 and assuming a ≠ 0 produces a quintic equation of the form:
Solving quintic equations in terms of
radicals (nth roots) was a major problem in algebra from the 16th century, when
quartic equations were solved, until the first half of the 19th century, when the impossibility of such a general solution was proved with the
Finding roots of a quintic equation
roots (zeros) of a given polynomial has been a prominent mathematical problem.
quartic equations by
factorization into radicals can always be done, no matter whether the roots are rational or irrational, real or complex; there are formulae that yield the required solutions. However, there is no
algebraic expression (that is, in terms of radicals) for the solutions of general quintic equations over the rationals; this statement is known as the
Abel–Ruffini theorem, first asserted in 1799 and completely proved in 1824. This result also holds for equations of higher degrees. An example of a quintic whose roots cannot be expressed in terms of radicals is x5 − x + 1 = 0.
Some quintics may be solved in terms of radicals. However, the solution is generally too complicated to be used in practice. Instead, numerical approximations are calculated using a
root-finding algorithm for polynomials.
Some quintic equations can be solved in terms of radicals. These include the quintic equations defined by a polynomial that is
reducible, such as x5 − x4 − x + 1 = (x2 + 1)(x + 1)(x − 1)2. For example, it has been shown that
has solutions in radicals if and only if it has an integer solution or r is one of ±15, ±22440, or ±2759640, in which cases the polynomial is reducible.
As solving reducible quintic equations reduces immediately to solving polynomials of lower degree, only irreducible quintic equations are considered in the remainder of this section, and the term "quintic" will refer only to irreducible quintics. A solvable quintic is thus an irreducible quintic polynomial whose roots may be expressed in terms of radicals.
To characterize solvable quintics, and more generally solvable polynomials of higher degree,
Évariste Galois developed techniques which gave rise to
group theory and
Galois theory. Applying these techniques,
Arthur Cayley found a general criterion for determining whether any given quintic is solvable. This criterion is the following.
Given the equation
Tschirnhaus transformationx = y − b/5a, which depresses the quintic (that is, removes the term of degree four), gives the equation
Both quintics are solvable by radicals if and only if either they are factorisable in equations of lower degrees with rational coefficients or the polynomial P2 − 1024 z Δ, named Cayley's resolvent, has a rational root in z, where
Cayley's result allows us to test if a quintic is solvable. If it is the case, finding its roots is a more difficult problem, which consists of expressing the roots in terms of radicals involving the coefficients of the quintic and the rational root of Cayley's resolvent.
There are several parametric representations of solvable quintics of the form x5 + ax + b = 0, called the
During the second half of the 19th century, John Stuart Glashan, George Paxton Young, and
Carl Runge gave such a parameterization: an
irreducible quintic with rational coefficients in Bring–Jerrard form
is solvable if and only if either a = 0 or it may be written
where μ and ν are rational.
In 1994, Blair Spearman and Kenneth S. Williams gave an alternative,
The relationship between the 1885 and 1994 parameterizations can be seen by defining the expression
where a = 5(4ν + 3)/ν2 + 1. Using the negative case of the square root yields, after scaling variables, the first parametrization while the positive case gives the second.
The substitution c = −m/l5, e = 1/l in the Spearman-Williams parameterization allows one to not exclude the special case a = 0, giving the following result:
If a and b are rational numbers, the equation x5 + ax + b = 0 is solvable by radicals if either its left-hand side is a product of polynomials of degree less than 5 with rational coefficients or there exist two rational numbers l and m such that
Roots of a solvable quintic
A polynomial equation is solvable by radicals if its
Galois group is a
solvable group. In the case of irreducible quintics, the Galois group is a subgroup of the
symmetric groupS5 of all permutations of a five element set, which is solvable if and only if it is a subgroup of the group F5, of order 20, generated by the cyclic permutations (1 2 3 4 5) and (1 2 4 3).
If the quintic is solvable, one of the solutions may be represented by an
algebraic expression involving a fifth root and at most two square roots, generally
nested. The other solutions may then be obtained either by changing the fifth root or by multiplying all the occurrences of the fifth root by the same power of a
primitive 5th root of unity, such as
In fact, all four primitive fifth roots of unity may be obtained by changing the signs of the square roots appropriately; namely, the expression
where , yields the four distinct primitive fifth roots of unity.
It follows that one may need four different square roots for writing all the roots of a solvable quintic. Even for the first root that involves at most two square roots, the expression of the solutions in terms of radicals is usually highly complicated. However, when no square root is needed, the form of the first solution may be rather simple, as for the equation x5 − 5x4 + 30x3 − 50x2 + 55x − 21 = 0, for which the only real solution is
An example of a more complicated (although small enough to be written here) solution is the unique real root of x5 − 5x + 12 = 0. Let a = √2φ−1, b = √2φ, and c = 4√5, where φ = 1+√5/2 is the
golden ratio. Then the only real solution x = −1.84208... is given by
More generally, if an equation P(x) = 0 of prime degree p with rational coefficients is solvable in radicals, then one can define an auxiliary equation Q(y) = 0 of degree p – 1, also with rational coefficients, such that each root of P is the sum of p-th roots of the roots of Q. These p-th roots were introduced by
Joseph-Louis Lagrange, and their products by p are commonly called
Lagrange resolvents. The computation of Q and its roots can be used to solve P(x) = 0. However these p-th roots may not be computed independently (this would provide pp–1 roots instead of p). Thus a correct solution needs to express all these p-roots in term of one of them. Galois theory shows that this is always theoretically possible, even if the resulting formula may be too large to be of any use.
It is possible that some of the roots of Q are rational (as in the first example of this section) or some are zero. In these cases, the formula for the roots is much simpler, as for the solvable
de Moivre quintic
where the auxiliary equation has two zero roots and reduces, by factoring them out, to the
such that the five roots of the de Moivre quintic are given by
where yi is any root of the auxiliary quadratic equation and ω is any of the four
primitive 5th roots of unity. This can be easily generalized to construct a solvable
septic and other odd degrees, not necessarily prime.
Other solvable quintics
There are infinitely many solvable quintics in Bring-Jerrard form which have been parameterized in a preceding section.
Up to the scaling of the variable, there are exactly five solvable quintics of the shape , which are (where s is a scaling factor):
Paxton Young (1888) gave a number of examples of solvable quintics:
An infinite sequence of solvable quintics may be constructed, whose roots are sums of nth
roots of unity, with n = 10k + 1 being a prime number:
There are also two parameterized families of solvable quintics:
The Kondo–Brumer quintic,
and the family depending on the parameters
cubic equations, there are solvable quintics which have five real roots all of whose solutions in radicals involve roots of complex numbers. This is casus irreducibilis for the quintic, which is discussed in Dummit.: p.17 Indeed, if an irreducible quintic has all roots real, no root can be expressed purely in terms of real radicals (as is true for all polynomial degrees that are not powers of 2).
Bring radical for details on these solutions and some related ones.
Application to celestial mechanics
Solving for the locations of the
Lagrangian points of an astronomical orbit in which the masses of both objects are non-negligible involves solving a quintic.
More precisely, the locations of L2 and L1 are the solutions to the following equations, where the gravitational forces of two masses on a third (for example, Sun and Earth on satellites such as
Gaia and the
James Webb Space Telescope at L2 and
SOHO at L1) provide the satellite's centripetal force necessary to be in a synchronous orbit with Earth around the Sun:
Using Kepler's Third Law and rearranging all terms yields the quintic
Solving these two quintics yields r = 1.501 x 109m for L2 and r = 1.491 x 109m for L1. The
Sun–Earth Lagrangian pointsL2 and L1 are usually given as 1.5 million km from Earth.
If the mass of the smaller object (ME) is much smaller than the mass of the larger object (MS), then the quintic equation can be greatly reduced and L1 and L2 are at approximately the radius of the
Hill sphere, given by:
That also yields r = 1.5 x 109m for satellites at L1 and L2 in the Sun-Earth system.