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Projection (linear algebra)
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===Product of projections=== The product of projections is not in general a projection, even if they are orthogonal. If two projections [[commuting matrices|commute]] then their product is a projection, but the [[converse (logic)|converse]] is false: the product of two non-commuting projections may be a projection. If two orthogonal projections commute then their product is an orthogonal projection. If the product of two orthogonal projections is an orthogonal projection, then the two orthogonal projections commute (more generally: two self-adjoint [[endomorphism]]s commute if and only if their product is self-adjoint).
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