sand dollar. n. 1. Any of various thin circular echinoderms of the order Clypeasteroida, especially Echinarachnius parma, of coastal northern Atlantic and Pacific waters, having a covering of short movable spines.
One reason why linear algebra is very useful is that linear transforms of normal variables are normal. If x ~ N(a, A), then Bx+b ~ N(Ba+b, BABT). In a way, when everything is linear and normal, everything is nice and analytically tractable.
Linear transformation is a function between two linear spaces over the same field of scalars, which is additive and homogeneous. Linear operator is a linear transformation for which the domain and the codomain spaces are the same and, moreover, in both of them the same basis is considered.