# Function that refuses to return a value

I wanted to know the exact solution to the following differential equation

\frac{\mathrm{d}^2 y}{\mathrm{d} t^2} = - \frac{1}{y-1}, \quad \begin{aligned} y(0) &= 0 \\ \left. \frac{\mathrm{d} y}{\mathrm{d} t} \right|_{t = 0} &= 0 \end{aligned}

I expected some function that would increase with time until it hits one.

So I set up the DE in MMA

ClearAll[y];
DSolve[{y''[t] == -(1/(y[t] - 1)), y == 0, y' == 0}, y[t], t]


And the result is

{y[t] -> E^-InverseErf[-Sqrt[(2/\[Pi])] t]^2 (-1 + E^
InverseErf[-Sqrt[(2/\[Pi])] t]^2)}


So I try to plot it with derivatives

y[t] := E^-InverseErf[-Sqrt[(2/\[Pi])] t]^2 (-1 + E^
InverseErf[-Sqrt[(2/\[Pi])] t]^2);
Plot[Evaluate[{y[t], 1/(y[t] - 1), D[y[t], t], D[y[t], {t, 2}]}], {t,
0, 2}]


And everything goes fine.

However, when I'm trying to obtain the value at which the function stops (I think it is $$t = \sqrt{\pi/2}$$), MMA doesn't know

Solve[y[t] == 1, t]
{}


When I tried to get certain values, MMA doesn't even return a number!

y[0.5]
y[0.5]


or

Evaluate[y[0.5]]
y[0.5]


Clearly, the plot behaves well around $$t = 0.5$$, so why does it refuse to return a value?

The only thing that seems to work is the following

y[t] /. t -> 0.5
0.12777


Can someone please explain this behaviour? I am curious.

• Simply y[t_] :=...., i.e. use the underscore in defining the function. Apr 22, 2019 at 21:44

When you write:

y[t]:=rhs


That means, "when you see y[t], rewrite it as rhs and continue evaluating. Here, t is just a literal symbol, not an argument, so y[anything but t] doesn't match and nothing happens. But:

y[t_]:=rhs


is very different. Here, t is the name of a pattern, unrestricted in this case, so it matches anything at all. The rewrite here starts by rewriting every t in rhs by whatever matched the pattern. The result replaces y[whatever], and evaluation continues. This is how Mathematica, an expression rewriting language, imitates a function call.

• Thank you very much! That might explain why the ./t-> thing worked! Apr 22, 2019 at 22:30