Numerical approximation of coefficients in fractions

asked 2018-07-26 03:50:29 +0100

Isamu
11 ●1 ●3

updated 2018-07-26 08:50:47 +0100

I am aware that for expressions in the type $$eq = c_0 + c_1x + c_2x^2...$$ the coefficients of x can be expressed as decimals by doing eq.polinomial(RR) however, I noticed that if it is in the form $$eq = \dfrac{c_0 + c_1x}{c_2 + c_3x}$$ or in any form where it is impossible to express as $c_0 + c_1x^n$ where n is some power of x, the eq.polinomial(RR) only returns an error giving TypeError: fraction must have unit denominator. How can I approximate $eq = \dfrac{c_0 + c_1x}{c_2 + c_3x}$ where $c_0, c_1, c_2, c_3$ becomes some decimals? I am aware that $\dfrac{c_0 + c_1x}{c_2 + c_3x}$ is not a polynomial however I do not know what it is.

answered 2018-07-26 09:56:33 +0100

rburing

10929 ●4 ●80 ●218 https://www.rburing.nl/

updated 2018-07-26 10:02:24 +0100

The result of expr.polynomial(RR) is a polynomial ring element. Dividing two polynomials gives an element of the fraction field of the polynomial ring (which are called rational functions). You can do e.g.

sage: expr = (pi + e*x)/(e + pi*x)
sage: expr.numerator().polynomial(RR) / expr.denominator().polynomial(RR)
(2.71828182845905*x + 3.14159265358979)/(3.14159265358979*x + 2.71828182845905)

Better yet, you can construct the fraction field yourself so you can use conversion:

sage: K = FractionField(PolynomialRing(RR, 'x'))
sage: K(expr)
(2.71828182845905*x + 3.14159265358979)/(3.14159265358979*x + 2.71828182845905)

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