Another problem with FunctionExpand
- To: mathgroup at smc.vnet.net
- Subject: [mg27841] Another problem with FunctionExpand
- From: Jack Goldberg <jackgold at math.lsa.umich.edu>
- Date: Mon, 19 Mar 2001 17:51:21 -0500 (EST)
- Sender: owner-wri-mathgroup at wolfram.com
Hi; Well here I go again. In my previous posts on FunctionExpand, Bob Hanlon suggested that using assumptions Element[x,Reals] in this expression: FunctionExpand[UnitStep[x^2],Element[x,Reals]] corrects the embarrassment of having FunctionExpand[UnitStep[x^2]] return UnitStep[-x]+UnitStep[x] which is incorrect when x=0. (Since UnitStep is defined only for real x, such an assumption is logically redundant.) However Element[x,Reals] does not save FunctionExpand[UnitStep[1-x^2],Element[x,Reals]] This call returns -UnitStep[-1+x]+UnitStep[1+x] (which is incorrect at x=1) on ver 4.0 using Unix and Mac operating systems. I believe my list of 4 failures (some quit modest, I admit) of the composition of FunctionExpand and UnitStep needs repair. The overwhelming response to my previous posts (1- it would have been zero except for Bob Hanlon's lone voice) lead me to suspect that this repair has low priority :-). Oh well, you are warned! ps. Here is another "barn-burner :-) - FunctionExpand[ f[x]*Discrete[x],Element[x,Reals] should return f[0]*Discrete[x]. It does not; f[x]*Discrete[x] is returned. Always peaking under the rug, Jack