RE: Replacement problem

*To*: mathgroup at smc.vnet.net*Subject*: [mg36926] RE: [mg36906] Replacement problem*From*: "DrBob" <drbob at bigfoot.com>*Date*: Wed, 2 Oct 2002 03:32:14 -0400 (EDT)*Reply-to*: <drbob at bigfoot.com>*Sender*: owner-wri-mathgroup at wolfram.com

Look at the FullForm of your expressions: FullForm[Sqrt[X + Y + Z]] Power[Plus[X, Y, Z], Rational[1, 2]] FullForm[f] Plus[Times[Rational[1, 4], C, Power[F, -2], Power[Plus[X, Y, Z], Rational[-1, 2]]], Times[B, Plus[A, Power[Plus[X, Y, Z], Rational[1, 2]]]]] FullForm[h] Plus[Times[B, Plus[A, Power[Plus[X, Y, Z], Rational[1, 2]]]], Times[C, Power[Plus[Times[4, Power[F, 2]], Power[Plus[X, Y, Z], Rational[1, 2]]], -1]]] The square root appears in two different forms! A solution is to replace both patterns: f = B*(A + Sqrt[X + Y + Z]) + C/(Sqrt[X + Y + Z]/4*F^2); f /. {Sqrt[X + Y + Z] -> Q, 1/Sqrt[X + Y + Z] -> 1/Q} (4*C)/(F^2*Q) + B*(A + Q) This is very annoying, of course, and it may not take care of every case. Looking at the FullForm should help, when it fails. Bobby -----Original Message----- From: Carlos Felippa [mailto:carlos at colorado.edu] To: mathgroup at smc.vnet.net Subject: [mg36926] [mg36906] Replacement problem These expressions are condensation of larger ones (about 700 lines or so each) but they illustrate random substitution failures in 4.2. Question: how can the substitution Sqrt[...]->Q always be made to work? The help file under ReplaceAll, ReplaceRepeated, etc, does not address this problem. Thanks f=B*(A+Sqrt[X+Y+Z])+C/(Sqrt[X+Y+Z]/4*F^2); Print[(f/.Sqrt[X+Y+Z]->Q)//InputForm]; B*(A + Q) + (4*C)/(F^2*Sqrt[X + Y + Z]) (* fails *) g=B*(A+Sqrt[X+Y+Z])+C/(Sqrt[X+Y+Z]*4*F^2); Print[(g/.Sqrt[X+Y+Z]->Q)//InputForm]; B*(A + Q) + C/(4*F^2*Sqrt[X + Y + Z]) (* fails *) h=B*(A+Sqrt[X+Y+Z])+C/(Sqrt[X+Y+Z]+4*F^2); Print[(h/.Sqrt[X+Y+Z]->Q)//InputForm]; B*(A + Q) + C/(4*F^2 + Q) (* works *)