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$5/2 = 2 \frac{1}{2}$ since both are the groupoid cardinality of the following action:

image

Thinking about this, it is actually quite enlightening. For more information, see the wonderful paper "From Finite Sets to Feynman Diagrams" by John Baez and James Dolan.
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$5/2 = 2 \frac{1}{2}$ since both are the groupoid cardinality of the following action:

image

Thinking about this, it is actually quite enlightening. For more information, see the wonderful paper "From Finite Sets to Feynman Diagrams" by John Baez and James Dolan.