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Definition df-irdg 5843
Description: Define a recursive definition generator on On (the class of ordinal numbers) with characteristic function 𝐹 and initial value 𝐼. This rather amazing operation allows us to define, with compact direct definitions, functions that are usually defined in textbooks only with indirect self-referencing recursive definitions. A recursive definition requires advanced metalogic to justify - in particular, eliminating a recursive definition is very difficult and often not even shown in textbooks. On the other hand, the elimination of a direct definition is a matter of simple mechanical substitution. The price paid is the daunting complexity of our rec operation (especially when df-recs 5807 that it is built on is also eliminated). But once we get past this hurdle, definitions that would otherwise be recursive become relatively simple. In classical logic it would be easier to divide this definition into cases based on whether the domain of g is zero, a successor, or a limit ordinal. Cases do not (in general) work that way in intuitionistic logic, so instead we choose a definition which takes the union of all the results of the characteristic function for ordinals in the domain of g. This means that this definition has the expected properties for increasing and continuous ordinal functions, which include ordinal addition and multiplication.

Note: We introduce rec with the philosophical goal of being able to eliminate all definitions with direct mechanical substitution and to verify easily the soundness of definitions. Metamath itself has no built-in technical limitation that prevents multiple-part recursive definitions in the traditional textbook style. (Contributed by Jim Kingdon, 19-May-2019.)

Assertion
Ref Expression
df-irdg rec(𝐹, 𝐼) = recs((g V ↦ (𝐼 x dom g(𝐹‘(gx)))))
Distinct variable groups:   x,g,𝐹   x,𝐼,g

Detailed syntax breakdown of Definition df-irdg
StepHypRef Expression
1 cF . . 3 class 𝐹
2 cI . . 3 class 𝐼
31, 2crdg 5842 . 2 class rec(𝐹, 𝐼)
4 vg . . . 4 setvar g
5 cvv 2533 . . . 4 class V
6 vx . . . . . 6 setvar x
74cv 1372 . . . . . . 7 class g
87cdm 4238 . . . . . 6 class dom g
96cv 1372 . . . . . . . 8 class x
109, 7cfv 4796 . . . . . . 7 class (gx)
1110, 1cfv 4796 . . . . . 6 class (𝐹‘(gx))
126, 8, 11ciun 3609 . . . . 5 class x dom g(𝐹‘(gx))
132, 12cun 2893 . . . 4 class (𝐼 x dom g(𝐹‘(gx)))
144, 5, 13cmpt 3770 . . 3 class (g V ↦ (𝐼 x dom g(𝐹‘(gx))))
1514crecs 5806 . 2 class recs((g V ↦ (𝐼 x dom g(𝐹‘(gx)))))
163, 15wceq 1373 1 wff rec(𝐹, 𝐼) = recs((g V ↦ (𝐼 x dom g(𝐹‘(gx)))))
Colors of variables: wff set class
This definition is referenced by:  rdgeq1  5844  rdgeq2  5845  rdgfun  5846  rdgexggg  5849  rdgi0g  5851  rdgifnon  5852  rdgivallem  5853  rdg0  5861
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