Factorial equations issues

It’s my pleasure!

please help me

Regarding the original question of Valeriu Ungureanu I get the following results

In[1]:= eq1 = x! == ((7!)!)/(7!)

Out[1]= x! == \
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In[2]:= Solve[
 x! == 898662748881423000174453419447084561929984926188022983951261503\
3222787930473384853142526156382655365373645838937078633930969099193251\
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000000000000000000000000000 && x \[Element] Integers, {x}]

Out[2]= {{x -> 5039}}

The version of Mathematica I use is:

In[3]:= $Version

Out[3]= "10.4.1 for Microsoft Windows (64-bit) (April 11, 2016)"

Thank you, Michael!
It works fine if we state that x is integer.

In[8]:= eq = x! == (7!)!/7!;
Solve[eq && x \[Element] Integers, x]
Solve[eq, x, Integers]
$Version

Out[9]= {{x -> 5039}}

Out[10]= {{x -> 5039}}

Out[11]= "10.4.1 for Microsoft Windows (64-bit) (April 11, 2016)"

Unfortunately, I don’t now the answer and I don’t want to make a supposition! Sorry, Mohamad!

Dear, Mohamad,

I can suggest you an answer to your question! Observe the graph of the function \Gamma^{-1}. It’s enough to look at the picture you posted in the first comment of this discussion. The function \Gamma^{-1} is a multi-valued (set-valued) function. So, the mathematical domain/area of such functions is larger than the traditional mono-valued calculus and as it seems you can’t have a simple representation (formula) of the frunction \Gamma^{-1} as you want.

Dear Valeriu

Thank you for your time. well, I think ??¹ is Multivalued function without restriction of ?.

but I restrict ? to => x Real : x?c.

then:

  1. ? is injective
  2. ? is surjective
  3. ? is single-valued
  4. so: ??¹ is single-valued.

Please let me know what you think.

Dear Mohamad,

Did you use a Mathematica code to plot the graphs in the first comment? If so, can you expose that code?

sure

In the complex domain there seem to be other solutions near the integer solution 5039:

Reduce[Gamma[1 + x] == (7!)!/7! && Abs[5039 - x] <= 2, x] // N

Marvelous!

In[3]:= Reduce[Gamma[1 + x] == (7!)!/7! && Abs[5039 - x] <= 10, x] // N

During evaluation of In[3]:= Reduce::nint: Warning: Reduce used numeric integration to show that the solution set found is complete. >>

Out[3]= x == 5039. || x == 5039. - 0.737025 I || 
 x == 5039. + 0.737025 I || x == 5039. - 1.47405 I || 
 x == 5039. + 1.47405 I || x == 5039. - 2.21108 I || 
 x == 5039. + 2.21108 I || x == 5039. - 2.9481 I || 
 x == 5039. + 2.9481 I || x == 5039. - 3.68513 I || 
 x == 5039. + 3.68513 I || x == 5039. - 4.42215 I || 
 x == 5039. + 4.42215 I || x == 5039. - 5.15918 I || 
 x == 5039. + 5.15918 I || x == 5039. - 5.8962 I || 
 x == 5039. + 5.8962 I || x == 5039. - 6.63323 I || 
 x == 5039. + 6.63323 I || x == 5039. - 7.37025 I || 
 x == 5039. + 7.37025 I || x == 5039. - 8.10728 I || 
 x == 5039. + 8.10728 I || x == 5039. - 8.8443 I || 
 x == 5039. + 8.8443 I || x == 5039. - 9.58133 I || 
 x == 5039. + 9.58133 I

In[26]:= Solve[Gamma[x + 1] == (7!)!/7! && Abs[x - 5039] <= 10,
x] // N

During evaluation of In[26]:= Solve::nint: Warning: Solve used numeric integration to show that the solution set found is complete. >>

Out[26]= {{x -> 5039.}, {x -> 5039. - 0.737025 I}, {x -> 
   5039. + 0.737025 I}, {x -> 5039. - 1.47405 I}, {x -> 
   5039. + 1.47405 I}, {x -> 5039. - 2.21108 I}, {x -> 
   5039. + 2.21108 I}, {x -> 5039. - 2.9481 I}, {x -> 
   5039. + 2.9481 I}, {x -> 5039. - 3.68513 I}, {x -> 
   5039. + 3.68513 I}, {x -> 5039. - 4.42215 I}, {x -> 
   5039. + 4.42215 I}, {x -> 5039. - 5.15918 I}, {x -> 
   5039. + 5.15918 I}, {x -> 5039. - 5.8962 I}, {x -> 
   5039. + 5.8962 I}, {x -> 5039. - 6.63323 I}, {x -> 
   5039. + 6.63323 I}, {x -> 5039. - 7.37025 I}, {x -> 
   5039. + 7.37025 I}, {x -> 5039. - 8.10728 I}, {x -> 
   5039. + 8.10728 I}, {x -> 5039. - 8.8443 I}, {x -> 
   5039. + 8.8443 I}, {x -> 5039. - 9.58133 I}, {x -> 
   5039. + 9.58133 I}}

Unfortunately, this is not a code! I obtained the same result by applying a plain English query: = inverse(gamma(x))

It’s interesting that an appropriate query: =Inverse[Gamma[x]], gives other result without plotting the graph of \Gamma^{-1} function.

I wanted to plot the graph of \Gamma^{-1} function to show that the issue related to multi-value function remains in the case when x\geq c \geq 0.

What I highlighted in this discussion is that to construct a graph of a inverse function is very easy… We must only change the axis (“transpose” the graph), what I wanted to do. But to construct the inverse of a function is a difficult problem…

So, the problem you posed remains, but the situation is much more clear…

Solution plot…

t = x /. Solve[Gamma[x + 1] == (7!)!/7! && Abs[x - 5039] <= 10, x] // N // Quiet;
ListPlot[(Tooltip[{Re[#], Im[#]}] &) /@ t, AspectRatio -> 1]

Could it possibly be periodic?

Plot[Log@Abs[Gamma[5039 + I y + 1] - (7!)!/7!], {y, -20, 20}, 
 PlotPoints -> 100]

I made an experiment to find if there exists some periodicity… And found something very interesting… The real part of the solutions is increasing, e.g. {5050.53 - 999.855 I, 5050.53 + 999.855 I} are two conjugate solutions with real parts greater than initial integer solution 5039. More the more, we can observe that there exists something that looks appropriate to periodicity, but, unfortunately it is not or it is a very “special” periodicity. This may be observed in the results that I obtained after doing an experiment with the following code.

t = x /. Solve[Gamma[x + 1] == (7!)!/7! && Abs[x - 5039] <= 1000, x] // N
ListPlot[(Tooltip[{Re[#], Im[#]}] &) /@ t, AspectRatio -> 1]

As it took some time to obtain the results, I attached the nb file with the final solutions.

P.S. The second file contains results for Abs[x-5039] \leq 1100 with 2988 solutions…