void problem(){
	gStyle->SetOptStat(1111);
	gStyle->SetOptFit(1111);
	double ta = 32;
	double tb = 40;

	string OutFileName = "fitted_problem.root";
	string InFileName = "problem.root";
	
	cout << "input  file name: " << InFileName << endl;
	cout << "output file name: " << OutFileName << endl;

	// open and create files
	cout << endl;
	cout << "######## reading files ########" << endl;
	TFile* InFile = new TFile(InFileName.c_str(),"OPEN");
	TTree* InTree = (TTree*) InFile->Get("tree");
	char muonID[40]; 
	InTree->SetBranchAddress("muonID", &muonID);
	
	
	
	TFile* OutFile = new TFile(OutFileName.c_str(),"RECREATE");
	OutFile->cd();
	double chi2_ndf_init;
	double chi2_ndf;
	TTree* OutTree = new TTree("tree","a tree");
	OutTree->Branch("muonID", &muonID, "muonID/C");
	OutTree->Branch("chi2_ndf_init", &chi2_ndf_init, "chi2_ndf_init/D");
	OutTree->Branch("chi2_ndf", &chi2_ndf, "chi2_ndf/D");

	int N = 5;
	cout << endl;
	cout << "######## start looping ########" << endl;
	cout << " index     muonID   fit chi2/ndf (32,40) us" << endl;
	for(int i=0; i<N; i++){
		InTree->GetEntry(i);
		InFile->cd();
		TGraph* gr_x = (TGraph*) InFile->Get(Form("gr_x_%s",muonID));
		TGraph* gr = (TGraph*) gr_x->Clone();

		
// 		TimeBounds st_end = GetStartEndTimePts(gr, ta, tb);
// 		int st = st_end.start_bin;
// 		int end = st_end.end_bin;
// 		vector <double>  ans = GetMinMaxMeanFromGraphRange(gr,st, end);
// 		double min_val = ans[0];
// 		double max_val = ans[1];
// 		double xe = ans[2];
// 		double ampl_A  = abs(max_val - min_val)/2;


		TF1* fit_func = new TF1(Form("fit_func_%s",muonID), "[0] + ([1]+[2]*x + [3]*x*x ) * cos([4]*x + [5]) + ([6]+[7]*x + [8]*x*x ) * cos([9]*x + [10])",ta, tb);
		fit_func->SetNpx(500000);
		fit_func->SetLineColor(2);
		fit_func->SetParameters(1, 2, 0.0, 0.0, 42, 0, 0, 0, 0, 39.8, 0);
		fit_func->SetParNames("xe","A","A1","A2","#omega_{c}","#phi_{c}","B","B1","B2","#omega_{x}","#phi_{x}");

		OutFile->cd();
		gr->Fit(fit_func,"REMQ");


		OutFile->cd();
		OutTree->Fill();
		gr->Write();

		delete gr;
		delete gr_x;
		
		//delete fit_func;
		}

	
	cout << endl;
	cout << "looping ended." << endl;
	
	cout << endl;
	cout << "######## finalizing ########" << endl;
	OutFile->cd();
	OutTree->Write();
	OutFile->Close();
	
	InFile->Close();

	cout << "Ciao" << endl;
	exit(0);
}