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Is there an attribute for symbols that makes NumericQ give True? For example Pi and E are symbols that behave this way, but I can't figure out which attribute to give it.

NumericFunction works well provided the symbol takes at least one argument:

SetAttributes[f,NumericFunction]


NumericQ[f]
(*False   <--- I want this to be true*)

NumericQ[f[1]]
(*True*)

But I need something for just symbols.

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I'm not sure if this is the official way to do it, but at least in 8.0.0.0 you can simply assign it:

NumericQ[f] = True

For example:

N[twopi, n_] := N[2*Pi, n]
NumericQ[twopi] = True
{twopi, twopi > 1}
(*
==> {twopi, True}
*)
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