search for: k2diff

The multiple exponential problem you are attempting has a well-known and long history. Lanczos 1956 book showed that changing the 4th decimal in a data set changes the parameters hugely. Nevertheless, if you just need a "fit" and not reliable paramters, you could reparameterize to k1 and k2diff=k2-k1, so k2=k1+kdiff. Then kdiff has a lower bound of 0, though putting 0 will almost certainly get you into computational trouble. You probably want an upper bound on k1 too. The problem is discussed in the book I published with Mary Walker-Smith in 1987, but I think our treatment is still only a...