Mathematica-like transformations rules

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You can use the substitute or subsmethods:

sage: a, b, x, y = SR.var('a b x y')
sage: expr = x^2 + sin(x) - x*y + exp(sin(y)-x)
sage: expr
x^2 - x*y + e^(-x + sin(y)) + sin(x)
sage: expr.subs({x: a, y: b})
a^2 - a*b + e^(-a + sin(b)) + sin(a)

See also:

• expr.subs
• expr.substitute_function
• expr.substitute
• expr.substitution_delayed

To get the documentation for those, define expr as above and type for example one of:

• expr.substitute_function?
• browse_sage_doc(expr.substitute_function)

One will give you text-mode help, and the other will render in html in a browser.