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How to assume that a function is positive?

• To: mathgroup at smc.vnet.net
• Subject: [mg114294] How to assume that a function is positive?
• From: Sam Takoy <sam.takoy at yahoo.com>
• Date: Tue, 30 Nov 2010 04:04:07 -0500 (EST)

Hi,

Who do I let Mathematica know that a function f is positive for all 
arguments? For example, how do I make the following work (I think my 
intention is clear):

Assuming[f[x] > 0, (f[x + y]^2)^(1/2) // Simplify]

Many thanks in advance,

Sam