# Problem with Derivative/Series and InverseFunction

Here is the Mathematica code I'm asking about:

fn = InverseFunction[ Log@#+Exp@#& ];
Print[  fn'                 [1.]]
Print[  D[fn[x],x]      /.x->1. ]
Print[  D[fn[x],{x,2}]  /.x->1. ]
Print[  fn''                [1.]]
Print[  D[D[fn[x],x],x] /.x->1. ]
Print[ Normal @ Series[fn@x, {x, 1., 1}] ]
Print[ Normal @ Series[fn@x, {x, 1., 2}] ]


Try it online!

As you can see:

fn is a function defined using InverseFunction. We can assume that this function has no closed-form.

It has first-order and second-order numerical derivative at the point 1, being 0.27614597273176456 and 0.04511431748742336 respectively.

However, calculate double derivative using Derivative (fn''[1.]) or Series (degree $\geq 2$) does not behave as expected, resolves to InverseFunction' object.

So, what's the best way to get the Series of that (and other similar) function?

• Answers about the bug in particular (and whether is that a bug) is also welcome. Dec 3, 2017 at 14:18

InverseFunction and Derivative[1] operate on functions. So, supply them with functions:

h=.
f = InverseFunction[h];
f'[x]


Derivative[1][h][InverseFunction[h][x]]^(-1)

h = x \[Function] Log[x] + Exp[x];
f'[x]


1/(E^InverseFunction[Function[x, Log[x] + Exp[x]]][x] + 1/ InverseFunction[Function[x, Log[x] + Exp[x]]][x])

Now also Series works. I use machine precision numbers in order to make the output readible:

Series[f[x], {x, 1., 4}] // TeXForm


$$0.512222+0.276146 (x-1.)+0.0225572 (x-1.)^2-0.0123554 (x-1.)^3-0.0000787926 (x-1.)^4+O\left((x-1.)^5\right)$$

• In conclusion: Anonymous function (#&) doesn't work, while explicit function (Function[y,y]) does work. Why? Is that a bug? They are both functions. Dec 3, 2017 at 14:00
• Also the wrap around InverseFunction is unnecessary - just InverseFunction[...] instead of Function[x, InverseFunction[...][x]] works, InverseFunction is supposed to return a function. Dec 3, 2017 at 14:02
• I have cleaned it up a bit. To my own surprise, lambda calculus seems not work. As a mathematician, I prefer Function anyway. It was actually the one thing that convinced me to use Mathematica... Dec 3, 2017 at 14:06
• If I were you, I would file that as a bug to Wolfram support. My impression used to be that lambda calculus and Function are supposed to be equivalent... Dec 3, 2017 at 14:09
• Moreover, I'd like to say that it would be more transparent for others if you would simply copy small code snippets directly into StackExchange... Dec 3, 2017 at 14:32

The problem with using an anonymous pure function inside of InverseFunction is that you get nested anonymous pure functions when evaluating the second derivative. Compare the following 2 TracePrint outputs:

TracePrint[InverseFunction[Function[x, Log[x]+Exp[x]]]'', Derivative[1][_]];
TracePrint[InverseFunction[Log[#]+Exp[#]&]'', Derivative[1][_]];


((1/(Function[x,Log[x]+Exp[x]]^[Prime])[InverseFunction[Function[x,Log[x]+Exp[x]]][#1]]&)^[Prime])

((1/((Log[#1]+Exp[#1]&)^[Prime])[InverseFunction[Log[#1]+Exp[#1]&][#1]]&)^[Prime])

Notice how the the first trace output has a mixture of x and #1, while the second trace output replaces x with #1. The latter trace is an example of a nested pure function using anonymous pure functions, which is why it doesn't work. The issue of nested anonymous pure functions has come up before, see:

So, what's the best way to get the Series of that (and other similar) function?

Instead of taking the series of the inverse function, you can apply InverseSeries to the series expansion of the function. For example:

InverseSeries @ Series[Log[x]+Exp[x], {x, 1, 2}] //TeXForm


$1+\frac{x-e}{1+e}-\frac{(e-1) (x-e)^2}{2 (1+e)^3}+O\left((x-e)^3\right)$

Series[
InverseFunction[Function[x, Log[x]+Exp[x]]][x],
{x, E, 2}
] //TeXForm


$1+\frac{x-e}{1+e}+\frac{(1-e) (x-e)^2}{2 (1+e)^3}+O\left((x-e)^3\right)$

For clarity, I used an expansion point of 1 for the function, which corresponds to an expansion point of E for the inverse function. To get the expansion of the inverse function around 1 you could do:

pt = x /. First @ Solve[Log[x] + Exp[x]==1 && 0<x<1, x];
Simplify[
InverseSeries @ Series[Log[x] + Exp[x], {x, a, 1}],
Exp[a]+Log[a]==1
] /. a->pt //TeXForm


$\operatorname{Root}\left[\left\{e^{\#1}+\log (\#1)-1\&,0.5122224330332299481607517669702611673420.304437036448245\right\}\right]+\frac{x-1}{e^{\operatorname{Root}\left[\left\{e^{\#1}+\log (\#1)-1\&,0.5122224330332299481607517669702611673420.304437036448245\right\}\right]}+\frac{1}{\operatorname{Root}\left[\left\{e ^{\#1}+\log (\#1)-1\&,0.5122224330332299481607517669702611673420.304437036448245\right\}\right]}}+O\left((x-1)^2\right)$

which yields the same result:

Series[
InverseFunction[Function[x, Log[x]+Exp[x]]][x],
{x, 1, 1}
] //TeXForm


$\operatorname{Root}\left[\left\{e^{\#1}+\log (\#1)-1\&,0.512222433033229948160786720184257682769052150649727599399330.\right\}\right]+\frac{x-1}{e^{\operatorname{Root}\left [\left\{e^{\#1}+\log (\#1)-1\&,0.512222433033229948160786720184257682769052150649727599399330.\right\}\right]}+\frac{1}{\operatorname{Root}\left[\left\{e^{\#1}+\log (\#1)-1\&,0.512222433033229948160786720184257682769052150649727599399330.\right\}\right]}}+O\left((x-1)^2\right)$

• Thanks! - So it's Mathematica implementation (use pure function to represent derivatives) that makes this impossible? Dec 3, 2017 at 16:38
• Isn't this a bug with handling pure functions by Derivative? Dec 3, 2017 at 20:08