Functions which have derivatives of all orders at all points and which, together with their derivatives,
fall off at least as rapidly as 
as 
, no matter how large 
is.
Particularly Well-Behaved Functions
See also
Regular SequenceExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Particularly Well-Behaved Functions." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/ParticularlyWell-BehavedFunctions.html