Page "Spin network" Paragraph 5
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Given any closed spin network, a non-negative integer can be calculated which is called the norm of the spin network.
Norms can be used to calculate the probabilities of various spin values.
A network whose norm is zero has zero probability of occurrence.
The rules for calculating norms and probabilities are beyond the scope of this article.
However, they imply that for a spin network to have nonzero norm, two requirements must be met at each vertex.
Suppose a vertex joins three units with spin numbers a, b, and c. Then, these requirements are stated as:
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