In the button theory, a topology subfield, the crossing number of a node is the minimum number of crossings of any button diagram of this node. The crossing number is a button variant.
To give an example; The trivial node has a crossing number of zero, the clover leaf node a crossing number of three and the number 8 knot a crossing number of four. There are no other knots with a crossing number of four or less. Only two knots have a crossing number of 5. As the crossing number rises, the number of knots with that crossing rate increases rapidly.
Primer buttons are traditionally indexed based on their intersection number, with a subscript indicating which particular node is meant by this amount of intersections (this sub-order is not based on anything except except that torus nodes are listed first). The summary goes from 31 (the clover leaf knot), 41 (the number eight knot), 51, 52 to 61, etc. This order has not changed since P. G. Tait published a table of buttons in 1877. Footnotes
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