check if symbolic expression contains a variable

asked 2012-05-04 14:38:37 +0100

leonardo
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updated 2012-05-05 21:43:53 +0100

kcrisman
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check if symbolic expression contains a given variable

Is there a function or method that tests a symbolic expression <type 'sage.symbolic.expression.expression'=""> to see if it contains a particular variable?

For example to test y(x) == -3/(x^3 + 3c + 3x) to see if it contains the variable c.

(I'm trying to write a a program to automatically plot the return value of the differential equation function desolve(), and it would help to be able to detect if the return expression contains the 'arbitrary constant' c.)

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answered 2012-05-04 19:55:00 +0100

Mike Hansen

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You can also use the has() method for an expression:

sage: var('x,c')
sage: y(x)=-3/(x^3 + 3*c + 3*x)
sage: y.has(c)
True

example on aleph.sagemath.org

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Is there an extension of this to functions? For example,

var('x,c')
function('f')
y(x) = -3/(x^3 + 3*c + 3*x) + f(c)
y.has(f(c))

? I am aware that this does not work as written, but I wonder if it is possible to extend it.

jupsal ( 2016-07-26 23:03:08 +0100 )edit

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answered 2012-05-04 16:09:08 +0100

Jason Grout
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updated 2012-05-04 16:09:29 +0100

Try the .variables() method for an expression:

var('x,c')
y(x)=-3/(x^3 + 3*c + 3*x)
print y.variables()
c in y.variables()

http://aleph.sagemath.org/?q=aefb061f...

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Thanks, Mike and Jason. Both methods work very nicely in my program.

leonardo ( 2012-05-05 14:02:59 +0100 )edit

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