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Integrate $\int\sin^{-1}\frac{2x}{1+x^2}.dx$

$$\sin^{-1}\frac{2x}{1+x^2}=\begin{cases}2\tan^{-1}x&\text{ if }|x|\leq{1}\\\pi-2\tan^{-1}x&\text{ if }|x|>{1}&\text{ and }x>0\\-\pi-2\tan^{-1}x&\text{ if }|x|>{1}&\text{ and }x<0\end{cases}$$

$$ \int\sin^{-1}\frac{2x}{1+x^2}.dx=\begin{cases}\int2\tan^{-1}xdx&\text{ if }|x|\leq{1}\\\int\pi dx-\int2\tan^{-1}xdx&\text{ if }|x|>{1}&\text{ and }x>0\\-\int\pi dx-\int2\tan^{-1}xdx&\text{ if }|x|>{1}&\text{ and }x<0\end{cases}=\begin{cases}2x\tan^{-1}x-\log(1+x^2)+C&\text{ if }|x|\leq{1}\\\pi x-2x\tan^{-1}x-\log(1+x^2)+C&\text{ if }|x|>{1}&\text{ and }x>0\\-\pi x-2x\tan^{-1}x-\log(1+x^2)+C&\text{ if }|x|>{1}&\text{ and }x<0\end{cases}$$

How do I obtain the case by case result for this type of indefinite integral ?

I tried 

Integrate[ArcSin[2*x/(1 + x^2)], x]

which is not giving the required result.

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  • $\begingroup$ Is $\sin^{-1}x$ supposed to be $\asin x$? That would be really helpful to differentiate from $(\sin x) ^{-1}$ $\endgroup$
    – LLlAMnYP
    May 15, 2018 at 16:37
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    $\begingroup$ @LLlAMnYP $\sin^{-1}\equiv\arcsin$ $\endgroup$
    – Sooraj S
    May 15, 2018 at 16:38
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    $\begingroup$ Many would disagree :-) $\endgroup$
    – LLlAMnYP
    May 15, 2018 at 17:10
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    $\begingroup$ Probably is not exactly what you were looking for, but you can try breaking the integral in the three different cases: FullSimplify[Integrate[ArcSin[(2 x)/(1 + x^2)], x], -1 <= x <= 1] FullSimplify[Integrate[ArcSin[(2 x)/(1 + x^2)], x], x > 1] FullSimplify[Integrate[ArcSin[(2 x)/(1 + x^2)], x], x < -1] $\endgroup$
    – Fraccalo
    May 15, 2018 at 17:12
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    $\begingroup$ @LLlAMnYP why 'd that be ?. $(\sin x)^{-1}$ is not same as $\sin^{-1}x$. Rather $(\sin x)^{-1}=\frac{1}{\sin x}$ and $\sin^{-1}x=\arcsin x$. I think this is how it is commonly defined. $\endgroup$
    – Sooraj S
    May 15, 2018 at 17:19

1 Answer 1

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FullSimplify[ComplexExpand[ArcSin[(2 x)/(1 + x^2)], TargetFunctions->{Re, Im}], x ∈ Reals]
Integrate[% /. Abs -> RealAbs, x]
% // Simplify
Resolve[ForAll[x, Evaluate[ArcSin[(2 x)/(1 + x^2)] == D[%, x]]], Reals]

ArcTan[Abs[-1 + x^2], 2 x]

$\begin{array}{cc} \{ & \begin{array}{cc} x \tan ^{-1}\left(x^2-1,2 x\right)+\log \left(x^2+1\right) & x\leq -1 \\ x \tan ^{-1}\left(1-x^2,2 x\right)-\log \left(x^2+1\right)+2 \log (2) & -1<x\leq 1 \\ x \tan ^{-1}\left(x^2-1,2 x\right)+\log \left(x^2+1\right) & \text{True} \\ \end{array} \\ \end{array}$

$\begin{array}{cc} \{ & \begin{array}{cc} x \tan ^{-1}\left(1-x^2,2 x\right)-\log \left(x^2+1\right)+\log (4) & -1<x\leq 1 \\ x \tan ^{-1}\left(x^2-1,2 x\right)+\log \left(x^2+1\right) & \text{True} \\ \end{array} \\ \end{array}$

True

Another way: $$\sin^{-1}\frac{2x}{1+x^2}=\begin{cases}2\tan^{-1}x&\text{ if }|x|\leq{1}\\\pi-2\tan^{-1}x&\text{ if }|x|>{1}&\text{ and }x>0\\-\pi-2\tan^{-1}x&\text{ if }|x|>{1}&\text{ and }x<0\end{cases}=\begin{cases}2\tan^{-1}x&\text{ if }|x|\leq{1}\\2 \tan ^{-1}\left(\frac{1}{x}\right)&\text{ if }x > 1&\text{or } x < -1\end{cases}$$ This can be verified with Mathematica

FullSimplify[π - 2 ArcTan[x] == 2 ArcTan[1/x], {Abs[x] > 1, x > 0}]
FullSimplify[-π - 2 ArcTan[x] == 2 ArcTan[1/x], {Abs[x] > 1, x < 0}]
Reduce[(Abs[x] > 1 && x > 0) || (Abs[x] > 1 && x < 0), x, Reals]

True

True

x < -1 || x > 1

expr = ArcSin[(2 x)/(1 + x^2)];
sol = Solve[Reduce[{y == expr}, x, Reals] // Simplify, y, Reals(*,MaxExtraConditions -> All*)]
pw = y /. sol /. ConditionalExpression -> List // Piecewise
int = Integrate[pw, x] // Simplify
Resolve[ForAll[x, Evaluate[expr == D[int, x]]], Reals]

{{y -> ConditionalExpression[2 ArcTan[1/x], x > 1 || x < -1]},

{y -> ConditionalExpression[2 ArcTan[x], -1 < x < 0 || 0 < x < 1]}}

$\begin{array}{cc} \{ & \begin{array}{cc} -\log \left(x^2+1\right)+2 x \tan ^{-1}(x)+\log (4) & -1<x\leq 1 \\ \log \left(x^2+1\right)+2 x \tan ^{-1}\left(\frac{1}{x}\right) & \text{True} \\ \end{array} \\ \end{array}$

True

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  • $\begingroup$ If you want to avoid conversion to RealAbs, you can do Assuming[x ∈ Reals, PiecewiseExpand[ FullSimplify[ ComplexExpand[ArcSin[(2 x)/(1 + x^2)], TargetFunctions -> {Re, Im}]]]] and then Integrate[%, x] gives a piecewise result with 3 terms. $\endgroup$
    – Greg Hurst
    May 16, 2018 at 13:04

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