2020-06-08 22:24:47 +0200 received badge ● Popular Question (source) 2020-06-08 22:24:47 +0200 received badge ● Notable Question (source) 2018-06-21 14:55:52 +0200 received badge ● Supporter (source) 2018-06-21 14:55:50 +0200 received badge ● Scholar (source) 2018-06-21 14:14:17 +0200 received badge ● Student (source) 2018-06-20 22:33:54 +0200 asked a question How to substitute values for derivatives? 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 "", 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: Define each derivative to be a specific value and evaluate the expression, or Substitute variables for derivatives, which can in turn be substituted with values and the expression evaluated?