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===Covariants of a binary quadric===
The algebra of invariants of the quadratic form ''ax''<sup>2</sup> + 2''bxy'' + ''cy''<sup>2</sup> is a polynomial algebra in 1 variable generated by the discriminant ''b''<sup>2</sup> − 4''ac'' of degree 2. The algebra of covariants is a polynomial algebra in 2 variables generated by the discriminant together with the form ''f'' itself (of degree 1 and order 2). {{harv|Schur|1968|loc=II.8}} {{harv|Hilbert|1993|loc=XVI, XX}}
===Covariants of a binary cubic===
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