3 ms·
The "correct" (in the categorical sense) sum operation for sets is disjoint (tagged) union, not union. So A + A + A = A ∐ A ∐ A, which is isomorphic to 3*A = (3
by ndriscoll 2y ago
The "correct" (in the categorical sense) sum operation for sets is disjoint (tagged) union, not union. So A + A + A = A ∐ A ∐ A, which is isomorphic to 3*A = (3,A) (i.e. each element of A ∐ A ∐ A carries the information of a tag from 3 and an element of A). All is well.