Notes on the procedure

The programs Polynomial Functions  and Rational Functions  perform roughly the same tasks as the program Curve Discussion , but differ fundamentally in the procedure.

As well known, zeros, extrema and points of inflection are determined by calculating the zeros of ƒ(x), ƒ'(x), ƒ"(x) and ƒ'"(x).

In the program Curve Discussion  these are approximately determined by searching the function, or the derivatives, step by step in an interval for a change in sign. This numerical method can be applied to any function, but it is not a viable method for finding definition gaps.

The programs Polynomial Functions  and Rational Functions  are limited to polynomials with rational coefficients. This makes it possible to exactly calculate the rational zeros of the function or the derivatives and, in the case of fractional rational functions, to determine the definition gaps via the zeros of the denominator polynomial.

As the following examples show, both methods have advantages and disadvantages.

Example 1:

                 x2 + 4·x + 3               (x + 1)·(x + 3)
ƒ(x) = ———————— = —————————
               x4 + x3 - 6·x2            x2·(x - 2)·(x + 3)
			   
x1 = -3  Eliminable Gap L(-3 | 0,0444444 )
x2 =  0  Pole without change of sign
x3 =  2  Pole with change of sign

The definition gaps are   correctly determined in the program Rational Functions .
In the program Curve Discussion  however, x₁ and x₃ are not recognized and instead are incorrectly displayed as points of inflection. The reason for this is the change in sign of the second derivative.

Example 2:

ƒ(x) = 3*x^7 + 3*x^6 + 17*x^5 - 5*x^4 + 34*x^3
        - 10*x^2 - 16*x + 8
 
N1(-0,68­3 | 0 )

H1(-0,295987 | 10,9025 )
T1( 0,471495 | 1,9943 )

W1( 0,0992583 | 6,34628 )

The polynomial has no rational zeros and the degree of the polynomial is too high to be able to use the formulas of Cardano and Ferrari (see equations of the 4th degree).
In the program Polynomial Functions  therefore no zeros, extremes and inflection points are found.
The numerical method in the program Curve Discussion can help here.

The linear factor decomposition of ƒ(x) shows whether the results of the programs Polynomial Functions  and Rational Functions have to be checked with the Curve Discussion . If this is not complete and the degree of the remainder polynomial is greater than 4, further irrational zeros, extrema and turning points can exist.
In the programs Polynomial Functions  and Rational Functions , the function term ƒ(x) is automatically copied to the clipboard and can be inserted into the Curve Discussion  with Ctrl V or the Paste option in the local menu.