problem with Expand

*To*: mathgroup at smc.vnet.net*Subject*: [mg26241] problem with Expand*From*: Peter Pollner <pollner at physik.uni-marburg.de>*Date*: Wed, 6 Dec 2000 02:16:24 -0500 (EST)*Sender*: owner-wri-mathgroup at wolfram.com

hi, I got a strange answer from Expand in the following situation: fn1, pn1, fn and pn are polynomials upto order to in x. I want to check the identity: pn1 - pn - k/2*fn*fn = 0 + O(x^3) the answer depends on the usage of Expand: Expand[pn1 - pn - k/2*fn*fn] and Expand[pn1] - Expand[pn - k/2*fn*fn] are not the same (the first expansion contains O(x^2) terms)! Here is the full input: pn1 = p0*x - k/12*n(n + 1)(2n + 1)x^2p0^2 - k/2*n(n + 1)p0*f0*x^2 - k/2*(n + 1)x^2f0^2 fn1 = f0*x + (n + 1)p0*x - k/24*n(n + 1)(n + 1)(n + 2)x^2 p0^2 - k/6*n(n + 1)(n + 2)x^2p0*f0 - k/4*(n + 1)(n + 2)x^2f0^2 pn = p0*x - k/12*(n - 1)n(2(n - 1) + 1)x^2p0^2 - k/2*n(n - 1)p0*f0*x^2 - k/2*n x^2f0^2 fn = f0*x + n p0*x - k/24*(n - 1)n^2(n + 1)x^2 p0^2 - k/6*(n - 1)n(n + 1)x^2p0*f0 - k/4*n(n + 1)x^2f0^2 Expand[pn1 - pn - k/2*fn*fn] Expand[pn1] - Expand[pn - k/2*fn*fn] I am using Mathematica 4.0.1.0, Platform : X Is it a bug? Please send me answers to the email address: Peter.Pollner at Physik.Uni-Marburg.De Thanks Peter Pollner