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Published online by Cambridge University Press: 24 October 2008
On an algebraic surface f of order n in space of three dimensions, the canonical system | k | of curves is traced by all those surfaces π of order n − 4 which fulfil certain conditions of adjunction at the singularities of f: for example, which pass simply through the double curves and through the isolated tacnodes of f; which pass doubly through the isolated fourfold points of f; and so on. The assigned fixed points and curves of the adjoint surfaces ø at these singularities of f are not taken to be part of the canonical system | k |; but | k | may have unassigned fixed parts e. Three cases are usually (3) distinguished.