Knots and Links
《Knots and Links》 · 第 81–82 页 · §4.3
A 2-sphere S intersecting a link L transversely in exactly two points is called a factorising sphere; decomposing along S gives links L1, L2, and L is the product link with factors L1, L2 (L = L1 # L2). A factor of a link is a proper factor if it is not the trivial knot, and is not equal to the link itself. A link with proper factors is called composite.