Hello dear ROOT Forum,

I am trying to fit a Bright-Wigner function to a histogram, by defining it myself and using it as an input function.

```
// Create a Breight-wigner function to fit!
//User defined (not predefined like the gauss!!!)
std::string bw_A = "2*sqrt(2)*[0]*[1]*sqrt([0]*[0]*([0]*[0]+[1]*[1]))";
std::string bw_B = "3.14159*sqrt([0]*[0]+sqrt([0]*[0]*([0]*[0]+[1]*[1])))";
std::string bw_C = "(x*x-[0]*[0])*(x*x-[0]*[0]) + [0]*[0]*[1]*[1]";
//form the BW function
std::string bw = Form("[2]*((%s)/(%s))/(%s))", bw_A.c_str(), bw_B.c_str(), bw_C.c_str());// "[2]" is a normalization factor to the whole BW function, change it accordingly!
//Now, we create the actual BW fit function, but in this case we also specify starting points for *two of the parameters to give it a hand* (as the Breit-Wigner is much more complex than the Gaussian function)
TF1 bwFit("bwFit", bw.c_str(), 50.e3, 200.e3);
bwFit.SetParameter(0, 100.e3);
bwFit.SetParameter(1, 1.e3);
```

However, I am getting the following error!

```
<TFormula::Eval>: Formula is invalid and not ready to execute
sqrt is unknown.
```

How do I proceed?

ps: I have tried commenting out the ‘sqrt’ lines, and it gives a plot, albeit a nonsensical one but at least I am sure that the problem is those lines only.