nlsLM Function Query

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nlsLM Function Query

Hi all,

Apologies in advance for my ignorance but ... I need to be able to determine x from a list of specified y values using the nlsLM function.

Background is that I need to move a calculation from an existing software package into R. Equation is:

y = (A*x)/(B+x)+(C*x)/D+x)+E

I had assumed that I could use the LM algorithm to determine the parameters A-E from known standards, rearrange the equation to solve for x and then plug to he experimentally-determined y values of the unknown samples into the rearranged equation to get the x values of the unknowns.

However, this is not how the existing software works. From the vendor:

"...When inverse calculation is not specified [software name] does series of iterations to match the intersect. Same Levenberg-Marquardt algorithm is used. [software name] does iterations until it gets a match for 12 digits..."

Is there any way to do this using the nlsLM function?

Thanks in advance and I can only apologise if this is an offensively stupid question.