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
where z=z1+z2, y=y1+y2 and f−1(y1) and f−1(y2) are known functions as already found. Can we say that f−1(y)=f−1(y1)+f−1(y2)? or can we modify this idea to get somethign useful?
Answer
y=W(eax+b)−W(ecx+d)+zx
W(ecx+d)+y−zx=W(eax+b)
(W(ecx+d)+y−zx)eW(ecx+d)+y−zx=eax+b
(W(ecx+d)+y−zx)eW(ecx+d)ey−zx=eax+b
(W(ecx+d)+y−zx)ecx+dW(ecx+d)=e(a+z)x+b−y
W(ecx+d)+y−zx=e(a−c+z)x+b−d−yW(ecx+d)
(e(a−c+z)x+b−d−y−1)W(ecx+d)=y−zx
W(ecx+d)=y−zxe(a−c+z)x+b−d−y−1
ecx+d=(y−zxe(a−c+z)x+b−d−y−1)ey−zxe(a−c+z)x+b−d−y−1
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 a−c+z=0 .
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