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Logarithms and desolve

asked 2015-12-14 08:41:29 -0500

AndersM gravatar image

updated 2015-12-16 13:48:46 -0500

More questions about desolve results: Using

 y = function('y', x)
desolve(diff(y,x) == y*(100-y), y)

results in the solution:

-1/100*log(y(x) - 100) + 1/100*log(y(x)) == _C + x

But that solution does not work for y<100. But it is correct if you assume that it should be log(abs(y(x)-100)) in the first term. Is the abs() implicit?

(The log terms actually comes from the integral of 1/y and 1/(100-y) so it should be absolute values.)

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Testing:

assume(x<0)

integral(1/x,x)

And the result is log(x). Hm...

AndersM gravatar imageAndersM ( 2015-12-16 13:51:09 -0500 )edit

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answered 2015-12-16 21:29:06 -0500

kcrisman gravatar image

updated 2015-12-17 11:33:24 -0500

log is a multivalued function. Most CASes will return log(x) and not log(abs(x)) as the antiderivative of 1/x. So that is probably the situation here - it is true in the multivalued sense?

Edit: here's how to set that maxima variable - but you have to do it in the "calculus" copy of Maxima inside Sage.

sage: integral(1/x,x)
log(x)
sage: maxima.eval(" logabs:true")
'true'
sage: integral(1/x,x)
log(x)
sage: maxima_calculus.eval(" logabs:true")
'true'
sage: integral(1/x,x)
log(abs(x))
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I found that it is possible to set a option variable in Maxima to get the log(abs(x)) answer when doing integrals http://maxima.sourceforge.net/docs/ma... . Can you set that variable to True in Sage.

AndersM gravatar imageAndersM ( 2015-12-17 02:36:44 -0500 )edit

Thanks! It even worked in Sage Cell Server.

AndersM gravatar imageAndersM ( 2015-12-17 12:19:28 -0500 )edit

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Asked: 2015-12-14 08:41:29 -0500

Seen: 96 times

Last updated: Dec 17 '15