C++ fitting routines - libraries?

Hi there,

I am trying to find a C++ library with fitting functions that I can use to very efficiently fit a variety of user-defined functions (e.g. A + Bt + Ct^2sin(Dt+E)) to a set of data (essentially numbers as a function of time). I know there are the ROOT TF1 but, as this is for a pure C++ application, was wondering where I can find a pure C++ library with such functions.

Any help would be much appreciated.

Many thanks.

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