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Problem with base 2 logs and Floor
*To*: mathgroup at smc.vnet.net
*Subject*: [mg73001] Problem with base 2 logs and Floor
*From*: neillclift at msn.com
*Date*: Sun, 28 Jan 2007 01:11:55 -0500 (EST)
Hi,
When I use an expression like this:
r - Floor[Log[2, r]] - 1 /. r -> 4
I get precision errors in Mathematica 5.2. If I use an expression like
this:
N[r - Floor[Log[2, r]] - 1] /. r -> 4
I get a correct result of 1 and no errors. If I use an expression like
this I get the same result:
N[r - Ceiling[Log[2, r]] - 1] /. r -> 4
This is to be expected as Floor[Log[2, r]] = Ceiling[Log[2, r]] when
r is a power of two.
Unfortunatly the expessions diverge for r=8:
N[r - Floor[Log[2, r]] - 1] /. r -> 8 gives 5
N[r - Ceiling[Log[2, r]] - 1] /. r -> 8 gives 4
How can I get exacts results for expessions like this?
Thanks.
Neill.
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