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v2009.01.01 - Convex Optimization

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3.1. CONVEX FUNCTION 201<br />

A<br />

B 1<br />

Figure 61: 1-norm ball B 1 in R 3 . Ball boundary contains all points<br />

equidistant from the origin in 1-norm. Plane A is overhead (drawn<br />

truncated). If 1-norm ball is expanded until it kisses A (intersects ball<br />

only at boundary), then distance (in 1-norm) from the origin to A is<br />

achieved. Cartesian axes drawn for reference. Euclidean ball would be<br />

spherical in this dimension. Only were A parallel to two axes could there<br />

be a minimum cardinality Euclidean least norm solution. Yet 1-norm ball<br />

offers infinitely many (but not all) A-orientations resulting in a minimum<br />

cardinality solution. (∞-norm ball is a cube in this dimension.)

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