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An angle which is not a right angle or a multiple of a right angle.

A nonsingular linear map A:R^n->R^n is orientation-reversing if det(A)<0.

Two (or more) intersecting curves that are perpendicular at their intersection are said to be orthogonal.

Orthogonal contravariant and covariant satisfy g_(ik)g^(ij)=delta_k^j, where delta_j^k is the Kronecker delta.

The map x^' = x+1 (1) y^' = 2x+y+1, (2) which leaves the parabola x^('2)-y^'=(x+1)^2-(2x+y+1)=x^2-y (3) invariant.

A tensor g whose discriminant satisfies g=g_(11)g_(22)-g_(12)^2>0.

The product of any number of perspectivities.

The pseudosphere is the constant negative-Gaussian curvature surface of revolution generated by a tractrix about its asymptote. It is sometimes also called the tractroid, ...

A plane figure for which quadrature is possible is said to be quadrable.

The number theta in the quadric (x^2)/(a^2+theta)+(y^2)/(b^2+theta)+(z^2)/(c^2+theta)=1 is called the parameter.