Re: More on Positive[x]

*To*: mathgroup at smc.vnet.net*Subject*: [mg7920] Re: More on Positive[x]*From*: bruck at pacificnet.net (Ronald Bruck)*Date*: Mon, 21 Jul 1997 03:22:10 -0400*Organization*: University of Southern California*Sender*: owner-wri-mathgroup at wolfram.com

In article <5qkea7$97k at smc.vnet.net>, Raya Firsov-Khanin <raya at mech.ed.ac.uk> wrote: :Hi, : :Thanks to everybody for replying to my question on how to declare a :variable positive. I just want to add that if you declare : :Positive[x] ^= True : :Positive[x] gives you True. However, Positive[1/x] or Positive[x*y] (where :Positive[y] ^=True) remain unevaluated. So, to work with positive variables :one needs to formulate a few rules, namely : :Unprotect[Positive]; :Positive[1/x_] := Positive[x]; :Positive[x_*y_] := Positive[x]&&Positive[y]; :Positive[x_/y_] := Positive[x]&&Positive[y]; :Positive[x_^n_] := Positive[x]; :Protect[Positive]; One of the pitfalls of modifying a built-in function ;-) What happens if x and y are both NEGATIVE and you apply your second or third rules? What happens if x is negative and n is even, in your third rule? --Ron Bruck Now 100% ISDN from this address