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What's wrong with :
sage: log(1000,2).n()
9.96578428466209
Note that this isn't an exact answer (the exact answer is log(1000)/log(2)). But it's a reasonable numerical approximation of a quantity whose "exact" numerical expression would be infinite...
Further discussion deserves reading part III of this excellent book as a prerequisite...
 Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.
 
                
                Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.