check if symbolic expression contains a variable

leonardo
15 ●2 ●2 ●5

kcrisman
12242 ●42 ●136 ●255

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 12 years ago

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 ( 8 years ago )

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answered 12 years ago

Jason Grout
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updated 12 years ago

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 ( 12 years ago )

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