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Re: Derivatives D[ ] as Functions inside Tables; Need Help!
*To*: mathgroup at smc.vnet.net
*Subject*: [mg13182] Re: [mg13102] Derivatives D[ ] as Functions inside Tables; Need Help!
*From*: Hugh Walker <hwalker at gvtc.com>
*Date*: Mon, 13 Jul 1998 07:42:30 -0400
*Sender*: owner-wri-mathgroup at wolfram.com
siegman at ee.stanford.edu wrote:
"I'm trying to define a function f1[a,x] and its derivative f2[a,x],
where from my point of view x is the independent variable and "a" is
a parameter I'll change on different runs. So I write, as a simple
example:
Remove["Global`*"]
f1[a_,x_] :=a Cos[x] + a^2 Sin[x]
f1[a,x]
f2[a_,x_] := D[f1[a,x],x]
f2[a,x]
a=2
f1[a,x]
f2[a,x]
Table[{x, f1[a,x], f2[a.x]} // N, {x,0,4}] // TableForm
and everything looks fine -- except the function f2[a,x] will not
evaluate inside the Table[ ]. I don't understand this -- help in
understanding will be much appreciated."
*************************************
There seems to be two problems: (1), the iterator {x,0,4} defaults to
integer steps because it is given no stepsize; and (2),the derivitive
wrapped by the Table function in the must be evaluated befor a
numerical value of x is substituted. Try this. Keep everything the same
excepy the last statement. Replace it with
Table[{xx, f1[a,xx], f2[a.xx]}xx->x // N, {x,0,4,0.2}] // TableForm
Actually, the //N is redundant.
Regards.
Hugh Walker
Professsor Emeritus
University of Houston
Gnarly Oaks
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