[R] fitting cosine curve

[lily li]

I'm trying the different parameters, but don't know what the error is:
Error in nlsModel(formula, mf, start, wts) :
  singular gradient matrix at initial parameter estimates

Thanks for any suggestions.

On Tue, Jun 20, 2017 at 7:37 PM, Don Cohen <don-r-help at isis.cs3-inc.com>
wrote:

>
> If you know the period and want to fit phase and amplitude, this is
> equivalent to fitting a * sin + b * cos
>
>  > >>> > I don't know how to set the approximate starting values.
>
> I'm not sure what you meant by that, but I suspect it's related to
> phase and amplitude.
>
>  > >>> > Besides, does the method work for sine curve as well?
>
> sin is the same as cos with a different phase
> Any combination of a and b above = c * sin (theta + d) for
> some value of c and d and = e * cos (theta + f) for some value
> of e and f.
> Also for any c,d and for any e,f there is an a,b.
> the c and e are what I'm calling amplitude, the d and f are what
> I'm calling phase.
>

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