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Definition df-iseq 9522
Description: Define a general-purpose operation that builds a recursive sequence (i.e. a function on the positive integers  NN or some other upper integer set) whose value at an index is a function of its previous value and the value of an input sequence at that index. This definition is complicated, but fortunately it is not intended to be used directly. Instead, the only purpose of this definition is to provide us with an object that has the properties expressed by iseq1 9533 and iseqp1 9538. Typically, those are the main theorems that would be used in practice.

The first operand in the parentheses is the operation that is applied to the previous value and the value of the input sequence (second operand). The operand to the left of the parenthesis is the integer to start from. For example, for the operation  +, an input sequence  F with values 1, 1/2, 1/4, 1/8,... would be transformed into the output sequence  seq 1 (  +  ,  F ,  QQ ) with values 1, 3/2, 7/4, 15/8,.., so that  (  seq 1
(  +  ,  F ,  QQ ) `  1
)  =  1,  (  seq 1
(  +  ,  F ,  QQ ) `  2
)  = 3/2, etc. In other words,  seq M (  +  ,  F ,  QQ ) transforms a sequence  F into an infinite series.

Internally, the frec function generates as its values a set of ordered pairs starting at 
<. M ,  ( F `
 M ) >., with the first member of each pair incremented by one in each successive value. So, the range of frec is exactly the sequence we want, and we just extract the range and throw away the domain.

(Contributed by Jim Kingdon, 29-May-2020.)

Assertion
Ref Expression
df-iseq  |-  seq M
(  .+  ,  F ,  S )  =  ran frec ( ( x  e.  (
ZZ>= `  M ) ,  y  e.  S  |->  <.
( x  +  1 ) ,  ( y 
.+  ( F `  ( x  +  1
) ) ) >.
) ,  <. M , 
( F `  M
) >. )
Distinct variable groups:    x,  .+ , y    x, F, y    x, M, y    x, S, y

Detailed syntax breakdown of Definition df-iseq
StepHypRef Expression
1 c.pl . . 3  class  .+
2 cS . . 3  class  S
3 cF . . 3  class  F
4 cM . . 3  class  M
51, 2, 3, 4cseq 9521 . 2  class  seq M
(  .+  ,  F ,  S )
6 vx . . . . 5  setvar  x
7 vy . . . . 5  setvar  y
8 cuz 8700 . . . . . 6  class  ZZ>=
94, 8cfv 4932 . . . . 5  class  ( ZZ>= `  M )
106cv 1284 . . . . . . 7  class  x
11 c1 7044 . . . . . . 7  class  1
12 caddc 7046 . . . . . . 7  class  +
1310, 11, 12co 5543 . . . . . 6  class  ( x  +  1 )
147cv 1284 . . . . . . 7  class  y
1513, 3cfv 4932 . . . . . . 7  class  ( F `
 ( x  + 
1 ) )
1614, 15, 1co 5543 . . . . . 6  class  ( y 
.+  ( F `  ( x  +  1
) ) )
1713, 16cop 3409 . . . . 5  class  <. (
x  +  1 ) ,  ( y  .+  ( F `  ( x  +  1 ) ) ) >.
186, 7, 9, 2, 17cmpt2 5545 . . . 4  class  ( x  e.  ( ZZ>= `  M
) ,  y  e.  S  |->  <. ( x  + 
1 ) ,  ( y  .+  ( F `
 ( x  + 
1 ) ) )
>. )
194, 3cfv 4932 . . . . 5  class  ( F `
 M )
204, 19cop 3409 . . . 4  class  <. M , 
( F `  M
) >.
2118, 20cfrec 6039 . . 3  class frec ( ( x  e.  ( ZZ>= `  M ) ,  y  e.  S  |->  <. (
x  +  1 ) ,  ( y  .+  ( F `  ( x  +  1 ) ) ) >. ) ,  <. M ,  ( F `  M ) >. )
2221crn 4372 . 2  class  ran frec (
( x  e.  (
ZZ>= `  M ) ,  y  e.  S  |->  <.
( x  +  1 ) ,  ( y 
.+  ( F `  ( x  +  1
) ) ) >.
) ,  <. M , 
( F `  M
) >. )
235, 22wceq 1285 1  wff  seq M
(  .+  ,  F ,  S )  =  ran frec ( ( x  e.  (
ZZ>= `  M ) ,  y  e.  S  |->  <.
( x  +  1 ) ,  ( y 
.+  ( F `  ( x  +  1
) ) ) >.
) ,  <. M , 
( F `  M
) >. )
Colors of variables: wff set class
This definition is referenced by:  iseqex  9523  iseqeq1  9524  iseqeq2  9525  iseqeq3  9526  iseqeq4  9527  nfiseq  9528  iseqval  9530  iseqvalt  9532
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