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Default and head in function argument

• To: mathgroup at smc.vnet.net
• Subject: [mg111604] Default and head in function argument
• From: NP <nomplume69 at gmail.com>
• Date: Sat, 7 Aug 2010 01:31:25 -0400 (EDT)

Hi,

I know how to define a function so that it evaluates only when the
head of the argument is of a certain type:

Clear[f];
f[x_Integer?Positive] := x^3
f[3]
f[3.1]
f[-3]

27
f[3.1]
f[-3]

and I also know how to set default to an argument of a function like
so:

Clear[f]
f[x_:3]
f[]

27

How do I do both i.e., specify head and default value?

Thanks,

NP