We shall build this argument from a lemma about injections on standard finite types,
Thm* (f:(mk). Inj(m; k; f)) mk ,
and exploiting the connection between
Assume a class has size
show
Each direction of the equality, namely,
mk andkm ,
may be proved in the same way, so we generalize the goal slightly to
x,y:. (x ~ y) xy .
Once this is proved,
Thm* EquivRel X,Y:Type. X ~ Y .
So, it is enough to show that
The connection between
Thm* (f:(AB). Bij(A; B; f)) (A ~ B) .
And since being an injection is part of being a bijection, i.e.,
Def Bij(A; B; f) == Inj(A; B; f) & Surj(A; B; f) ,
Thm* (f:(mk). Inj(m; k; f)) mk .
QED
A corrolary isThm* a,b:. (a ~ b) a = b .
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