Thursday, 22 January 2015

Inverse function of y=W(eax+b)W(ecx+d)+zx



I have a simple question for which I am looking for a closed form expression (If there exits one). In other words, given:



y=W(eax+b)W(ecx+d)+zx




where W is the Lambert W function and a,b,c,d,z are some constants, what is the function f, such that
x=f(y)



Thanks alot in advance.



EDIT : If there exists no closed form solution, I will be happy to see nice arguments supporting this.



EDIT2 : As can be seen, we have a solution for the simplified version of this problem. If there exists a solution I have the following ideas to resolve the full version of the problem:



1- Is it possible to write W(ef(x))=W(eax+b)W(ecx+d)




2- Having y1=W(eax+b)+z1x

and y2=W(ecx+d)+z2x



where z=z1+z2, y=y1+y2 and f1(y1) and f1(y2) are known functions as already found. Can we say that f1(y)=f1(y1)+f1(y2)? or can we modify this idea to get somethign useful?


Answer



y=W(eax+b)W(ecx+d)+zx



W(ecx+d)+yzx=W(eax+b)



(W(ecx+d)+yzx)eW(ecx+d)+yzx=eax+b




(W(ecx+d)+yzx)eW(ecx+d)eyzx=eax+b



(W(ecx+d)+yzx)ecx+dW(ecx+d)=e(a+z)x+by



W(ecx+d)+yzx=e(ac+z)x+bdyW(ecx+d)



(e(ac+z)x+bdy1)W(ecx+d)=yzx



W(ecx+d)=yzxe(ac+z)x+bdy1




ecx+d=(yzxe(ac+z)x+bdy1)eyzxe(ac+z)x+bdy1



Then you can only force to use Lagrange inversion theorem or Lagrange reversion theorem unless the special cases when a=0 or c=0 or ac+z=0 .


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