# Complex dimension

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In mathematics, * complex dimension* usually refers to the dimension of a complex manifold

*M*, or a complex algebraic variety

*V*. If the complex dimension is

*d*, the

**real dimension**will be 2

*d*. That is, the smooth manifold

*M*has dimension 2

*d*; and away from any singular points

*V*will also be a smooth manifold of dimension 2

*d*.

However, for a real algebraic variety (that is a variety defined by equations with real coefficients), its dimension refers commonly to its complex dimension, and its real dimension refers to the maximum of the dimensions of the manifolds contained in the set of its real points. The real dimension is not greater than the dimension, but may be smaller. For example, the equation is a variety of (complex) dimension 2 (a surface), but of real dimension 0 — it has only one real point, (0, 0, 0).

The same points apply to codimension. For example a smooth complex hypersurface in complex projective space of dimension *n* will be a manifold of dimension 2(*n* − 1). A complex hyperplane does not separate a complex projective space into two components, because it has real codimension 2.
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