# InverseFunction's domain is not the range of the function that was inverted

I wrote a function to calculate how long of a period has past when a planet traverses an angle theta from perihelion in an orbit of a given eccentricity.

Integrate[(1 - 0.0167^2)^1.5/(2 Pi)  1/(1 + 0.0167 Cos [theta])^2, theta]

0.159088 (2.00084 ArcTan[0.983437 Tan[0.5 theta]] - (1.00028 Sin[theta]) /
(59.8802 + 1. Cos[theta]))


The domain around the origin of the formula above is -Pi ~ Pi so I defined

s[theta_] := formula above /; 0 <= theta < Pi;
s[theta_] := formula above + 1 /; Pi < theta <= 2Pi;


To get how the angle varies with time, I applied InverseFunction to s and got the inverse function theta[t]. When I input theta[0.25], I get a numerical result but a s^{-1}[0.99] when input theta[0.99].

Where's the point?

It's strange that InverseFunction[s][0.99] doesn't work here, but the following seems to work fine:

s = Function[theta, Piecewise[
{{0.159088*(2.00084*ArcTan[0.983437*Tan[0.5*theta]] - (1.00028*Sin[theta])/
(59.8802 + 1.*Cos[theta])), Inequality[0, LessEqual, theta, Less, Pi]},
{1 + 0.159088*(2.00084*ArcTan[0.983437*Tan[0.5*theta]] - (1.00028*Sin[theta])/
(59.8802 + 1.*Cos[theta])), Inequality[Pi, Less, theta, LessEqual, 2*Pi]}}, 0]];

InverseFunction[s] /@ {0.99, 0.25}

Out[27]= {6.21821, 1.60419}


If you want to inject your formula into the Function without having to copy-paste, this is how you can do it:

s = Block[{
formula := 0.159088 (2.00084 ArcTan[0.983437 Tan[0.5 theta]] - (1.00028 Sin[theta])/(59.8802 + 1. Cos[theta])),
theta
},
Function[theta,
Evaluate@Piecewise[{
{
formula,
0 <= theta < Pi
},
{
formula + 1,
Pi < theta <= 2 Pi
}
}
]
]
]

• Many thanks! It indeed works! I hope one day the problem can be solved. – Protesticon May 31 '18 at 14:01