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.

**Follow-Ups**:**Re: Problem with base 2 logs and Floor***From:*Andrzej Kozlowski <akoz@mimuw.edu.pl>