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The following expressions give True in the NumericQ test:

Map[NumericQ, {Rational[2, 3], Complex[2, 3], Root[1 + 2 #1 + #1^5 &, 1], RootSum[-7 - 3 #1 + #1^5 &, Sin[#1] &]}] (* Out: {True, True, True, True} *)

However the heads Rational, Complex, Root and RootSum do not have the attribute NumericFunction:

Attributes[Root] (* Out: {NHoldAll, Protected, ReadProtected} *)

... but NumericQ still returns True when applied to them.

Is there a way to obtain a list of all the possible heads h of expressions h[args] that do NOT have the NumericFunction attribute, and yet will give True in the test NumericQ[h[args]]?

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Note that Root is only considered numeric when its argument does NOT contain any non-numerical parameters. For instance: 

Root[(# - a) &, 1] (* Out: a *) NumericQ@Root[(# - a) &, 1] (* Out: False *)

A similar argument applies to RootSum, Rational, and Complex: 

NumericQ@RootSum[a + # &, Log[#1] &] (* False *) NumericQ@Rational[a, 2] (* False *) NumericQ@Complex[a, 2] (* False *)

It seems to me that these examples still fall within the general guideline presented in the documentation for NumericQ, under "Details": "NumericQ[expr] gives True whenever N[expr] would yield an explicit number".

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  • $\begingroup$ Yes they do fall within the documentation, what I want to know is the explicit list of ALL the heads that fall within the documentation, I know Root, RootSum, Rational, Complex, are there others? Could new ones appear in future versions of Mathematica? If so, how could they be automatically detected in the code of a Mathematica package? $\endgroup$ Commented Jun 9, 2016 at 21:58

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