I tried FunctionExpand
, but it does not do what you want:
Erf[1/2] // FunctionExpand
You can use a replacement rule instead:
erfRule =
Erf[x_] :> 2/Sqrt[\[Pi]] Inactive[Integrate][E^(-t^2), {t, 0, x}];
Erf[1/2] /. erfRule
The expression Integrate[E^(-t^2), {t, 0, 1/2}]
is a number, of course. To check it you can use NumericQ
, not NumberQ
:
NumberQ[Integrate[E^(-t^2), {t, 0, 1/2}]]
NumericQ[Integrate[E^(-t^2), {t, 0, 1/2}]]