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A three-dimensional coordinate system in which the axes satisfy the right-hand rule.

A pseudocylindrical map projection which distorts shape, area, scale, and distance to create attractive average projection properties.

dN^^+kappa_idr=0, where N^^ is the unit normal vector and kappa_i is one of the two principal curvatures.

|_n]={n for n!=0; 1 for n=0. (1)

The root lattice of a semisimple Lie algebra is the discrete lattice generated by the Lie algebra roots in h^*, the dual vector space to the Cartan subalgebra.

A rooted tree is a tree in which a special ("labeled") node is singled out. This node is called the "root" or (less commonly) "eve" of the tree. Rooted trees are equivalent ...

If the matrices A, X, B, and C satisfy AX-XB=C, then [I X; 0 I][A C; 0 B][I -X; 0 I]=[A 0; 0 B], where I is the identity matrix.

The divisibility test that an integer is divisible by 9 iff the sum of its digits is divisible by 9.

One of the straight lines sweeping out a ruled surface. The rulings on a ruled surface are asymptotic curves.

A scalar is a one-component quantity that is invariant under rotations of the coordinate system.