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Can Sage verify (some) hyperbolic identities?

asked 2015-07-24 14:35:00 +0100

f9qMRIktzYLG gravatar image

If I enter bool( cosh(x) == (exp(x)+exp(-x))/2 ), the output is False. This happens with other hyperbolic identities. Is it possible to verify them in Sage? If so, what is the correct way?

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answered 2015-07-24 21:54:31 +0100

eric_g gravatar image

updated 2015-07-26 10:01:42 +0100


As far as I know, there is no simple way to verify such identities with Sage yet. A related issue is that Sage is currently unable to simplify cosh(x) - (exp(x)+exp(-x))/2 to 0:

sage: (cosh(x) - (exp(x)+exp(-x))/2).simplify_full()
1/2*(2*cosh(x)*e^x - e^(2*x) - 1)*e^(-x)

But there is a workaround: the rewrite extension written by Fran├žois Maltey. To use it, download the file rewrite-20110123.sage from this page; then in a Sage session, you may ask to rewrite the hyperbolic functions in terms of exp, so that the outcome of the check is now True:

sage: %runfile rewrite-20110123.sage
sage: bool( rewrite(cosh(x), 'sinhcosh2exp') == (exp(x)+exp(-x))/2 )

Equivalently, you may also ask to rewrite the whole identity:

sage: bool( rewrite(cosh(x) == (exp(x)+exp(-x))/2, 'sinhcosh2exp') )

See here for the documentation of rewrite. Hopefully, it shall be included in main Sage some day.

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answered 2015-07-26 15:29:13 +0100

rws gravatar image

You might want to familiarize with the Maxima and SymPy parts of Sage. With Maxima you can do

sage: ex = cosh(x) - (exp(x)+exp(-x))/2
sage: ex._maxima_().exponentialize().sage()

Here the Maxima function exponentialize does the rewrite. In Sympy:

sage: ex = cosh(x) - (exp(x)+exp(-x))/2
sage: import sympy
sage: sympy.simplify(ex)
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Asked: 2015-07-24 14:35:00 +0100

Seen: 516 times

Last updated: Jul 26 '15