analysis (differential/integral calculus, functional analysis, topology)
metric space, normed vector space
open ball, open subset, neighbourhood
convergence, limit of a sequence
compactness, sequential compactness
continuous metric space valued function on compact metric space is uniformly continuous
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A real quadratic function is a quadratic function in the real numbers. Equivalently, it is a solution to the third-order linear homogeneous ordinary differential equation
with initial conditions
Given a real quadratic function , if , after completing the square, the quadratic function may be written as
The extremum occurs at
and the extremum is a minimum if and a maximum if . The value of at the extremum is
When , the real quadratic function is said to be concave up, while when , the real quadratic function is said to be concave down.
If , the real quadratic function is degenerate?; it becomes a real affine function.
In classical mathematics, the law of trichotomy holds in the real numbers, so the three cases above cover every real number. However, in constructive mathematics, trichotomy does not hold in the real numbers, and as a result, there exists real quadratic functions such that one cannot decide whether is concave up, concave down, or degenerate. Similarly, there exists real quadratic functions such that one cannot decide whether the extremum is a minimum, is a maximum, or does not exist due to the degeneracy of .
Given a real quadratic function , for real numbers , , such that , since is continuously differentiable if , due to the inverse function theorem, there are two branches of the partial inverse function of , and , such that if , then and have domain
and if , and have domain
In both cases, and are defined as solutions to the first-order nonlinear ordinary differential equation
with the initial conditions for the principal branch are
and the initial conditions for the reflected branch are
As a result, and are automatically pointwise continuously differentiable, and for every in the domain of and ,
Since this differential equation is separable, one could use separation of variables to see that and satisfy the functional equations and .
If the discriminant is greater than zero , then the quadratic function has two zeroes apart from each other. The larger of the two zero of is at and the smaller zero of is at . If the discriminant is less than or equal to zero , then zero isn’t in the domain of either and .
If , , and , then the real quadratic function above is the square function . The principal branch of the continuous partial inverse function of the square function is called the continuous principal square root , and is the solution to the differential equation
with initial condition . The reflected branch of the continuous partial inverse function of the square function is simply the negation of the principal square root .
For any quadratic function , the principal branch of its inverse has a canonical definition in terms of the continuous principal square root:
and the reflected branch has a canonical definition in terms of the continuous principal square root as well:
The continuous quadratic formula is the evaluation of the two branches at zero, if zero is in the domain of and :
The continuous quadratic formula is only valid for real quadratic functions with positive discriminant, unlike the quadratic formula in the discrete field of quadratic irrational numbers, which is valid for quadratic functions with rational coefficients and a non-negative discriminant. But there’s an analytic reason for why the quadratic formula should be restricted to real quadratic functions with positive discriminant: for any real quadratic function with a non-positive discriminant , there is no interval on the real numbers such that the conditions required for the intermediate value theorem are fulfilled: if , then for all real numbers , and if , then for all real numbers .
As stated above, when a real quadratic function has a non-positive discriminant, , if , then for all real numbers , and if , then for all real numbers . When has a negative discriminant, , then is provably always apart from zero: if , then for all real numbers , and if , then for all real numbers . As a result, has no zeroes in the real numbers if the determinant of is negative.
Now, suppose that has zero discriminant. This means that , and after completing the square, the resulting function is equal to
By definition of an ordered field, there are no multiplicative nilpotent elements in the real numbers whose absolute value is greater than zero, and thus every nilpotent element is equal to zero. As a result,
There is only one zero, which occurs at the extremum of the real quadratic function.
In classical mathematics, the law of trichotomy holds in the real numbers, so the three cases above cover every real number. However, in constructive mathematics, trichotomy does not hold in the real numbers, and as a result, there exists real quadratic functions such that one cannot decide whether the discriminant of is positive, negative, or zero. As a result, there exist real quadratic functions where one cannot decide the number of zeroes the function has. Furthermore, one cannot prove that a real quadratic function with positive determinant has exactly two zeroes.
Given a real quadratic function , for real numbers , , , where , suppose we want to approximate a zero of . There is an algorithm called Newton's method which would allow us to do so. The real quadratic function has discriminant and has 2 zeroes if , 1 zero at the extremum if , or no zeroes of . (In constructive mathematics, there are real quadratic functions where it isn’t possible to determine the number of zeroes the real quadratic function has.)
Assume that . Select a real number as the initial guess, where or , and the next guess would be defined as
Newton’s method isn’t valid when because of division by zero. When , Newton’s method converges towards the zero at , and when , Newton’s method converges towards the zero .
See also:
Last revised on August 21, 2024 at 01:52:45. See the history of this page for a list of all contributions to it.