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I was calculating the volume of Reuleaux Cube , something like Reuleaux Tetrahedron.

Evaluation

1/24 (-24+24 Sqrt[2]-36 ArcCos[(2 Sqrt[2])/3]+54 ArcCot[Sqrt[2]]-13 ArcSin[Sqrt[2/3]]+13 ArcSin[1/Sqrt[3]]+5 ArcTan[1/Sqrt[2]]-108 ArcTan[5/Sqrt[2]]-39 ArcTan[Sqrt[2]]-20 ArcTan[3-2 Sqrt[2]]+108 ArcTan[3+2 Sqrt[2]])

It was really complicated.

I know the answer can be written as $\sqrt{2}-1 + \frac{97 \pi }{12}-27 \arctan\left(\sqrt{2}\right)$ or $\sqrt{2}-1+\frac{4 \pi }{3}-\frac{9}{2}\arcsin \left(\frac{23}{27}\right)$.

I tried this way to simplify it:

ArcTogether[expr_] := FullSimplify[
  expr //. k_ (f : (ArcSin | ArcCos | ArcTan | ArcCot))[x_] :>
  ArcTan[TrigExpand@Tan[k f[x]]] + k f[x] - ArcTan[Tan[k f[x]]]]
ArcExpand[expr_] := First@SortBy[expr /. a : ArcTan[x_] :> Table[
  k ArcTan[z] /. First@Solve[{TrigExpand[Tan /@ (a == k ArcTan[z])], 
  a == k ArcTan[z]}, z, Reals], {k, 2, 20, 1}], LeafCount]

Then I use ArcExpand@ArcTogether[Last@ans] // FullSimplify and got:

$$V=\sqrt{2}-1-\frac{\pi }{6}+\frac{3}{2} \arctan\left(\frac{1633}{13870 \sqrt{2}}\right)$$

This still a bit complex. I need some more powerful way to simplify.


This code $8X$ faster than the original one:

reg=Reduce[{(x+1/2)^2+(y+1/2)^2+(z+1/2)^2<1,x>0,y>0,z>0}]
int=Sequence@@(reg/.{And->List,Inequality[a_,Less,x_,Less,b_]:>{x,a,b}});
AbsoluteTiming[8 Integrate[1,int]]//FullSimplify
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  • 2
    $\begingroup$ That's an octahedron you have there, not a cube. $\endgroup$ Nov 2, 2017 at 15:28
  • $\begingroup$ What mathematical argument do you have to believe a better simplification can be made? $\endgroup$
    – m_goldberg
    Nov 2, 2017 at 20:42
  • $\begingroup$ What is your measure of simplicity? $\endgroup$
    – bbgodfrey
    Nov 3, 2017 at 1:27

1 Answer 1

3
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expr = 1/24 (-24+24 Sqrt[2]-36 ArcCos[(2 Sqrt[2])/3]+54 ArcCot[Sqrt[2]]-13 ArcSin[Sqrt[2/3]]+13 ArcSin[1/Sqrt[3]]+5 ArcTan[1/Sqrt[2]]-108 ArcTan[5/Sqrt[2]]-39 ArcTan[Sqrt[2]]-20 ArcTan[3-2 Sqrt[2]]+108 ArcTan[3+2 Sqrt[2]]);

rule1 = k_ (f : (ArcSin | ArcCos | ArcTan | ArcCot))[x_] :> 
   ArcSin[TrigExpand@Sin[k f[x]]] + k f[x] - ArcSin[Sin[k f[x]]];

rule2 = a : (ArcSin[x_]) :> 
   First@SortBy[
     Cases[Flatten@
       Table[(# /. 
            Solve[{TrigExpand[Sin /@ (a == #)], a == #}, z, 
             Reals]) & /@ {k ArcSin[z], Pi - k ArcSin[z]}, {k, 2, 
         10}], _?NumericQ], StringLength@ToString@# + LeafCount@# &];

expr //. rule1 // FullSimplify
% /. rule2
% /. rule2
% /. rule2
% // Simplify

-1 + Sqrt[2] - Pi/8 + 1/24 (-Pi + 9 ArcSin[(34616649035551240 Sqrt[2])/150094635296999121])

-1 + Sqrt[2] - Pi/8 + 1/24 (-Pi + 36 ArcSin[1633/19683])

-1 + Sqrt[2] - Pi/8 + 1/24 (-Pi + 36 (Pi - 9 ArcSin[1/3]))

-1 + Sqrt[2] + (4 Pi)/3 - 27/2 ArcSin[1/3]

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