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Finding length in recursive definition?

• To: mathgroup at smc.vnet.net
• Subject: [mg60877] Finding length in recursive definition?
• From: "Jose Reckoner" <reckoner at gmail.com>
• Date: Sat, 1 Oct 2005 02:55:52 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

I have something like:
f[1] = 1
f[2] = 3
f[n_] := f[n] = f[n - 1] + f[n - 2]

and in the course of work, f[n] gets evaluated an unknown number of
times resulting in

>> ?f
f[1] = 1
f[2] = 3
f[3] = 4
f[n_] := f[n] = f[n - 1] + f[n - 2]

I want to figure out the greatest integer n such that f[n] has already
been computed and is stored. In this case, it is 3.

How can I do this?

Thanks!

Jose