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Definition df-rdg 6627
Description: Define a recursive definition generator on  On (the class of ordinal numbers) with characteristic function  F and initial value  I. This combines functions  F in tfr1 6617 and  G in tz7.44-1 6623 into one definition. 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. But once we get past this hurdle, otherwise recursive definitions become relatively simple, as in for example oav 6714, from which we prove the recursive textbook definition as theorems oa0 6719, oasuc 6727, and oalim 6735 (with the help of theorems rdg0 6638, rdgsuc 6641, and rdglim2a 6650). We can also restrict the  rec operation to define otherwise recursive functions on the natural numbers  om; see fr0g 6652 and frsuc 6653. Our  rec operation apparently does not appear in published literature, although closely related is Definition 25.2 of [Quine] p. 177, which he uses to "turn...a recursion into a genuine or direct definition" (p. 174). Note that the  if operations (see df-if 3700) select cases based on whether the domain of  g is zero, a successor, or a limit ordinal.

An important use of this definition is in the recursive sequence generator df-seq 11279 on the natural numbers (as a subset of the complex numbers), allowing us to define, with direct definitions, recursive infinite sequences such as the factorial function df-fac 11522 and integer powers df-exp 11338.

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 NM, 9-Apr-1995.) (Revised by Mario Carneiro, 9-May-2015.)

Assertion
Ref Expression
df-rdg  |-  rec ( F ,  I )  = recs ( ( g  e. 
_V  |->  if ( g  =  (/) ,  I ,  if ( Lim  dom  g ,  U. ran  g ,  ( F `  ( g `  U. dom  g ) ) ) ) ) )
Distinct variable groups:    g, F    g, I

Detailed syntax breakdown of Definition df-rdg
StepHypRef Expression
1 cF . . 3  class  F
2 cI . . 3  class  I
31, 2crdg 6626 . 2  class  rec ( F ,  I )
4 vg . . . 4  set  g
5 cvv 2916 . . . 4  class  _V
64cv 1648 . . . . . 6  class  g
7 c0 3588 . . . . . 6  class  (/)
86, 7wceq 1649 . . . . 5  wff  g  =  (/)
96cdm 4837 . . . . . . 7  class  dom  g
109wlim 4542 . . . . . 6  wff  Lim  dom  g
116crn 4838 . . . . . . 7  class  ran  g
1211cuni 3975 . . . . . 6  class  U. ran  g
139cuni 3975 . . . . . . . 8  class  U. dom  g
1413, 6cfv 5413 . . . . . . 7  class  ( g `
 U. dom  g
)
1514, 1cfv 5413 . . . . . 6  class  ( F `
 ( g `  U. dom  g ) )
1610, 12, 15cif 3699 . . . . 5  class  if ( Lim  dom  g ,  U. ran  g ,  ( F `  ( g `
 U. dom  g
) ) )
178, 2, 16cif 3699 . . . 4  class  if ( g  =  (/) ,  I ,  if ( Lim  dom  g ,  U. ran  g ,  ( F `  ( g `  U. dom  g ) ) ) )
184, 5, 17cmpt 4226 . . 3  class  ( g  e.  _V  |->  if ( g  =  (/) ,  I ,  if ( Lim  dom  g ,  U. ran  g ,  ( F `  ( g `  U. dom  g ) ) ) ) )
1918crecs 6591 . 2  class recs ( ( g  e.  _V  |->  if ( g  =  (/) ,  I ,  if ( Lim  dom  g ,  U. ran  g ,  ( F `  ( g `
 U. dom  g
) ) ) ) ) )
203, 19wceq 1649 1  wff  rec ( F ,  I )  = recs ( ( g  e. 
_V  |->  if ( g  =  (/) ,  I ,  if ( Lim  dom  g ,  U. ran  g ,  ( F `  ( g `  U. dom  g ) ) ) ) ) )
Colors of variables: wff set class
This definition is referenced by:  rdgeq1  6628  rdgeq2  6629  nfrdg  6631  rdgfun  6633  rdgdmlim  6634  rdgfnon  6635  rdgvalg  6636  rdgval  6637  rdgseg  6639  dfrdg2  25366
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