Constant term in polynomial?
- To: mathgroup at smc.vnet.net
- Subject: [mg3326] Constant term in polynomial?
- From: bruck at mtha.usc.edu (Ronald Bruck)
- Date: Mon, 26 Feb 1996 02:57:02 -0500
- Organization: University of Southern California, Los Angeles, CA
- Sender: owner-wri-mathgroup at wolfram.com
Arrgh, I feel stupid asking this question, but I can't think how to do it:
how do I find the constant term in a polynomial in several variables in
Mathematica? For example, the "7" in 7 + 3 x y + y^2 ?
I suppose one way would be to use
Coefficient[Coefficient[7 + 3 x y + y^2,x,0],y,0].
But that's incredibly clunky, especially since I may have fifty or more
variables in my real-life problem.
I could evaluate the expression under the rule {x->0, y->0}, with the same
problem: for fifty variables that's awkward. I could build the rule using
Variables[expr], but that's clumsy and seems inefficient.
First[7 + 3 x y + y^2] will work for this one, since the 7 is present and
appears first in the FullForm representation. But it won't work in
First[3 x y + y^2], which returns 3 x y.
OK, so I can build a command which computes Variables[First[expr]], and
if that's empty, returns 0; otherwise returns First[expr]. Also clunky
IMHO, but it seems the most workable--unless there's some trap I'm missing?
Or I can introduce an auxiliary variable "one", refer to the polynomial as
"7 one + 3 x y + y^2", and ask for Coefficient[expr, one]. Gag! If I ever
want to EVALUATE it, I have to remember to use the rule one->1.
There MUST be a standard way to do this, but I can't think of what it could be!
--Ron Bruck
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