[R] How to find the zero (only the real solution) with the package polynom ?

[Ptit_Bleu]

Hi again,

I found a solution (but maybe it is not the best one) : I converted the
poynom into a function.

pderiv<-as.function((deriv(polynomial(pfit$coefficients))))
uniroot(pderiv, c(0.8,0.2))[[1]]
[1] 0.4778413

Now I just have to find the corresponding "b" to this "a" but I think it
must be quite easy (am I right ?).

I don't delete my post as it maybe be helpful to someone else but if the
Administrator wants to delete it, it is ok to me.

See you,
Ptit Bleu.

-------------------



Ptit_Bleu wrote:
> 
> Hello,
> 
> I have 3 columns : a, b and a*b
> I would like to find the pair (a,b) so that a*b is the minimum but not
> from the points I measured but from the fit of the curve (I have more
> points that the ones given below but I fit only on this part because I 
> know that the minimum a*b is in this interval).
> 
> I thought doing it this way :
> 
> - to fit a*b=f(a)
> abfit<-lm(ab ~ poly(a,8,raw=T))
> 
> - to use the package polynom to make the polynom 
> library(polynom)
> polynomial(abfit$coefficients)
> 0.03537506 - 0.8506516*x + 8.302545*x^2 - 44.80418*x^3 + 144.2161*x^4 -
> 283.2458*x^5 + 331.1776*x^6 - 210.5144*x^7 + 55.86646*x^8
> 
> - to derive the polynom
> deriv(polynomial(pfit$coefficients))
> -0.8506516 + 16.60509*x - 134.4125*x^2 + 576.8643*x^3 - 1416.229*x^4 +
> 1987.066*x^5 - 1473.601*x^6 + 446.9317*x^7
> 
> - to find the zero
> solve(deriv(polynomial(pfit$coefficients)))
> [1] 0.1749897+0.0568886i 0.1749897-0.0568886i 0.3742571-0.1235393i
> 0.3742571+0.1235393i 0.4778418+0.0000000i 0.8604070+0.1306799i
> 0.8604070-0.1306799i
> 
> And here is my little problem : I don't want the complex solutions, only
> the real one (0.4778418).
> 
> I also tried uniroot but I got an error message :
> uniroot(deriv(polynomial(pfit$coefficients)), c(0.8,0.2))
> Error in uniroot(deriv(polynomial(abfit$coefficients)), c(0.8, 0.2)) : 
>         Impossible to find the function "f"
> 
> Is there a way to extract this real solution ?
> 
> I thank you in advance for your help,
> Have a nice day,
> Ptit Bleu. 
> 
> -------------------------------------------------
> Set of points
> a        b                    a*b
> 0.8	0.033320357	0.026656286
> 0.76	0.023930636	0.018187283
> 0.72	0.0153903	0.011081016
> 0.68	0.007844914	0.005334542
> 0.64	0.001598132	0.001022804
> 0.6	-0.00315939	-0.001895634
> 0.56	-0.006402796	-0.003585566
> 0.52	-0.008379886	-0.004357541
> 0.48	-0.009528793	-0.004573821
> 0.44	-0.010198446	-0.004487316
> 0.4	-0.010638282	-0.004255313
> 0.36	-0.010937536	-0.003937513
> 0.32	-0.011169004	-0.003574081
> 0.28	-0.011343229	-0.003176104
> 0.24	-0.0114878	-0.002757072
> 0.2	-0.011630636	-0.002326127
> 0.16	-0.011724586	-0.001875934
> 
> 

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