Re: Recombining CoefficientList
- To: mathgroup at smc.vnet.net
- Subject: [mg2531] Re: [mg2501] Recombining CoefficientList
- From: Allan Hayes <hay at haystack.demon.co.uk>
- Date: Thu, 16 Nov 1995 01:46:38 -0500
sherod at boussinesq.Colorado.EDU (Scott Herod)
in mathgroup at smc.vnet.net
writes
> CoefficientList[poly, {x1,x2,...,xn}] returns an array of
> coefficients of the various powers of the xi in poly. Has someone
> got a code segment to recreate poly from the list of xi and the
>output?
Here is a slight modification of tmj 5.3 p24 (In[] and Out[])
ToPolynomial[cl_, vars_] :=
Plus@@Flatten[
MapIndexed[#1 Times@@(vars^(#2-1))&,cl,{Depth[cl]-1}]
]
Example:
poly = a x^3 + b x^4 y + c x y + d y^2 + e
3 4 2
e + a x + c x y + b x y + d y
cl = CoefficientList[poly, {x,y}]
{{e, 0, d}, {0, c, 0}, {}, {a, 0, 0}, {0, b, 0}}
ToPolynomial[cl, {x,y}]
3 4 2
e + a x + c x y + b x y + d y
Allan Hayes
hay at haystack.demon.co.uk