Matlab custom fitting fails -

i have bunch of data have fit following formula: [x*sin(1/x)]^2. don't know coefficients pick such function, tried:

model = fittype('a*((xx/b)*sin(b/xx))^2+c','independent',{'xx'},'dependent',{'ff_norm(:,i)'}, 'coefficients', {'a','b','c'}); opt=fitoptions(model); myfit=fit(xx,ff_norm(:,i),model, opt); myfit plot(myfit,xx,ff_norm(:,i)) 

i fit looks straight line. know it's better enter start points well, have no idea how pick these because don't know how matlab interprets them , mean. when try numbers, it's giving me errors.

i not sure problem encounter, tried reproduce problem code:

xx = (0.1:0.1:10).'; error = rand(size(xx)).*2; data = 5.*((xx./6).*sin(6./xx)).^2+7+error; model = fittype('a.*((xx./b)*sin(b./xx)).^2+c','independent',{'xx'},'dependent'...     ,{'data'}, 'coefficients', {'a','b','c'}); opt = fitoptions(model); opt.startpoint = [1 1 1]; [myfit,gof] = fit(xx,data,model,opt) plot(myfit,xx,data); 

and gave me output:

myfit =       general model:      myfit(xx) = a.*((xx./b)*sin(b./xx)).^2+c      coefficients (with 95% confidence bounds):        =        5.18  (4.846, 5.514)        b =       6.318  (5.84, 6.796)        c =       7.995  (7.777, 8.213) 

which quite correct, , plot looks fine:

indeed, not works on first try, should try several times, until fine. way using while loop , gof (goodness of fit) output fit function, search adequate parameters model. result above:

gof =             sse: 27.757        rsquare: 0.91357            dfe: 97     adjrsquare: 0.91178           rmse: 0.53493 

this off course depend on how data have, fact true.