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| | The Disjoint Sum |
 | | Given two sets, A and B, we define their disjoint sum, A+B, to be the collection of symbols of form (a,) with a in A or of form (,b) with b in B. If A and B are ordered, we can extend their orders to A+B by deeming every (a,) to come before every (,b). |
 | | Given two functions (Af:C) and (Bg:D), we define their disjoint sum, f+g, to be (A+Bf+g:C+D) mapping any (a,) to (f(a),) and any (,b) to (,g(b)). |
 | | It should be noted that the standard embeddings of A and B in A+B compose with this to yield f and g as appropriate: in this sense, A+B is a minimal set via embeddings of A and B in which one may factorise any pair of functions, as above. |
| www.chaos.org.uk /~eddy/math/disjoint.html (603 words) |
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