# Declaring a positive variable [duplicate]

Why I have this behaviour?

R /: Greater[R, 0] = True
R > 0
=> True
Simplify[Sqrt[R^2]]
=> Sqrt[R^2]


I expect R as last result.

• You have to use assumptions: Simplify[Sqrt[R^2], Assumptions :> R > 0]. – Leonid Shifrin Mar 16 '13 at 15:14
• @LeonidShifrin: No way to say that R is positive once for all? – enzotib Mar 16 '13 at 15:26
• @LeonidShifrin: thanks for pointing to that question – enzotib Mar 16 '13 at 15:31
• It's definitely best to work with \$Assumptions, but if for some reason you don't want that, and want to avoid Simplify, then you could also just define R = Abs[r]. – Jens Mar 16 '13 at 15:42

assumptions = {R >= 0};

which returns R as expected.
• There is no way to attach that information to R, so to avoid to recall the Assumptions option each time? – enzotib Mar 16 '13 at 15:19