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answered 2015-12-22 21:31:24 +0100
One solution
sage: K.<I> = QuadraticField(-1) sage: R.<z> = K[] sage: S.<x> = R[] sage: q = x**2 - z**2 * I sage: q.discriminant() 4*I*z^2
The default I in Sage is (for now) a symbolic expression.