Re: Expand on RHS of rule

*To*: mathgroup at yoda.physics.unc.edu*Subject*: Re: Expand on RHS of rule*From*: TODD GAYLEY <TGAYLEY at ccit.arizona.edu>*Date*: Sat, 30 May 1992 17:32 MST

Athos Kasapi asks: > Has anybody found a way to work around the problem that > Expand does not work when it is on the right hand side > of a rule? For example, if I define > p := Sqrt[ (a + b) (c + d) ] > and then apply the rule > p /. Sqrt[ x_ y_ ] -> Sqrt[ Expand[ x y ] ] > Then all I get back is > Sqrt[ (a + b)(c + d) ] > What I wanted was > Sqrt[ ac + ad + bc + bd ] What is happening here is that the Expand[x y] is being evaluated before the substitution of values for x and y. What you need is the RuleDelayed operator (see p. 258): p /. Sqrt[x_ y_] :> Sqrt[Expand[x y]] ------------------------- Todd Gayley Department of Ecology and Evolutionary Biology University of Arizona tgayley at ccit.arizona.edu