Have a good day. What I want to do is to find the parameters δ0, δ2, δ4 to fit the data set "Nilson123" (1) to the "y" function (3). When I run the "FindFit" (4), I obtain the the parameters output as (5). I put these parameters to the y function (6) but I obtain a fit which is very bad (7). However when I put another parameter values δ0=0.611, δ2=2.00, δ4=0.026 from literature to the y function (8), I acheive a better fit. I couldn't understand why the parameters obtained with the FindFit function don't work. Is there anyone to help me?
