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How to substitute values for derivatives?

asked 2018-06-20 14:01:41 -0500

bob323 gravatar image

I would like to first differentiate a function, then substitute values for derivatives within the function.

For example, a function might be cos(theta)*sin(phi). I would like to differentiate with respect to another variable (time in this case):

sage: t = var('t')
sage: theta = function('theta')(t)
sage: phi = function('phi')(t)
sage: A = cos(theta)*sin(phi)
sage: C = diff(A,t,t)
sage: C
-cos(theta(t))*sin(phi(t))*diff(phi(t), t)^2 - 2*cos(phi(t))*sin(theta(t))*diff(phi(t), t)*diff(theta(t), t) - cos(theta(t))*sin(phi(t))*diff(theta(t), t)^2 + cos(phi(t))*cos(theta(t))*diff(phi(t), t, t) - sin(phi(t))*sin(theta(t))*diff(theta(t), t, t)
sage: C.subs(theta=5,phi=6,diff(theta(t),t)=7,diff(phi(t),t)=8,diff(theta(t),t,t)=9,diff(phi(t),t,t)=10)
  File "<ipython-input-147-f1b68bf72320>", line 1
    C.subs(theta=Integer(5),phi=Integer(6),diff(theta(t),t)=Integer(7),diff(phi(t),t)=Integer(8),diff(theta(t),t,t)=Integer(9),diff(phi(t),t,t)=Integer(10))
SyntaxError: keyword can't be an expression

I believe the error comes from trying to define the entire function theta(t), diff(phi(t),t,t), etc. as a single number. It should work just as well to substitute other variables in the place of the derivatives; I only want the derivatives there for getting the initial equations. Is there a way to either:

  1. Define each derivative to be a specific value and evaluate the expression, or
  2. Substitute variables for derivatives, which can in turn be substituted with values and the expression evaluated?
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answered 2018-06-20 16:19:35 -0500

Emmanuel Charpentier gravatar image

updated 2018-06-20 16:22:22 -0500

WorksForMe(TM):

sage: var("t")
t
sage: theta=function("theta")
sage: A=cos(theta(t))
sage: diff(A,t)
-sin(theta(t))*diff(theta(t), t)
sage: var("B,C")
(B, C)
sage: diff(A,t).subs([diff(theta(t),t)==B,theta(t)==C])
-B*sin(C)

But remember that in the subs method, the single equal sign (which is a marker for keywords in a function call) is accepted for a single substitition. To to multiple substitutions, use either a list of equations (whose sides are separated by ==, i. e. a double equal sign) or a dictionary.

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Asked: 2018-06-20 14:01:41 -0500

Seen: 27 times

Last updated: Jun 20