Re: What is f[1]? Advanced question

*To*: mathgroup at smc.vnet.net*Subject*: [mg131323] Re: What is f[1]? Advanced question*From*: John Doty <noqsiaerospace at gmail.com>*Date*: Fri, 28 Jun 2013 04:13:53 -0400 (EDT)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-outx@smc.vnet.net*Delivered-to*: mathgroup-newsendx@smc.vnet.net*References*: <20130626011408.DC7E86A2B@smc.vnet.net> <kqdshl$n34$1@smc.vnet.net>

On Thursday, June 27, 2013 4:25:04 AM UTC-6, amannucci wrote: > Thanks. This is very helpful. I remain puzzled by this: > > > > u // FullForm > > FullForm[Symbol["u"]] > > FullForm[Unevaluated[Symbol["u"]] > > Head[u] > > > > FullForm[{1, 2, 3}] > > Head[{1, 2, 3}] > > > > In other words, Head and FullForm seem consist for the list, and inconsistent for the variable u. u's full form is just, literally "u", not Symbol["u"]. But the list's FullForm starts with "List". Head of Symbol["u"] is not Symbol, but "u". What gives? If FullForm did what you expected, its output would be infinite, because every expression in Mathematica has a head. Remember that the head of an expression is itself an (arbitrary!) expression. So, FullForm "bottoms out" when further expansion would expose an elementary head like Symbol or Integer.

**References**:**What is f[1]? Advanced question***From:*amannucci <Anthony.J.Mannucci@jpl.nasa.gov>