# Linear fitter problems in ROOT v5-28-00-patches

While trying to fit a TGraph with a “pol3”, I get (note how it “jumps”):

[code]root [18] g->Fit(“pol3”)

Minimizer is Linear
Chi2 = 2075.61
NDf = 464
p0 = 4.89194e+06 +/- 753524
p1 = -7394.77 +/- 1142.6
p2 = 3.72553 +/- 0.577519
p3 = -0.000625518 +/- 9.72989e-05
(class TFitResultPtr)141375584
root [19] g->Fit(“pol3”)

Minimizer is Linear
Chi2 = 2099.3
NDf = 464
p0 = 2.10406e+06 +/- 495473
p1 = -3167.38 +/- 751.307
p2 = 1.58883 +/- 0.379741
p3 = -0.000265534 +/- 6.3978e-05
(class TFitResultPtr)137580960
root [20] g->Fit(“pol3”)

Minimizer is Linear
Chi2 = 15384.7
NDf = 464
p0 = 4.07854e+07 +/- 5.93457e+06
p1 = -61821.9 +/- 8998.9
p2 = 31.2352 +/- 4.54841
p3 = -0.00526026 +/- 0.000766302
(class TFitResultPtr)145594664
root [21] g->Fit(“pol3”)

Minimizer is Linear
Chi2 = 2262.49
NDf = 464
p0 = 8.40135e+06 +/- 1.03222e+06
p1 = -12716.3 +/- 1565.21
p2 = 6.41525 +/- 0.791122
p3 = -0.00107867 +/- 0.000133286
(class TFitResultPtr)140055624[/code]

I also tried the “Fit Panel”.
If “Robust” is 1.00, I get:
Warning in TLinearFitter::Fitting:: illegal value of H, default is taken
and the ROOT hangs (one needs “Ctrl-Z” and “kill %1”).
I also often get tons of:
Error in TDecompChol::Decompose(): matrix not positive definite
Error in TDecompChol::Solve(): Decomposition failed
Error in TLinearFitter::Linf: Matrix inversion failed
(and the ROOT quite often likes to hang in these cases)

Hi,

The problem with the robust fitter from the fit panel has been fixed before for 5.32. If this is not the case please let me know.