Definition df-seq 13365

Description: Define a general-purpose operation that builds a recursive sequence (i.e., a function on an upper integer set such as ℕ or ℕ₀) 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 seq1 13377 and seqp1 13379. 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 seq1( + , 𝐹) with values 1, 3/2, 7/4, 15/8,.., so that (seq1( + , 𝐹)‘1) = 1, (seq1( + , 𝐹)‘2) = 3/2, etc. In other words, seq𝑀( + , 𝐹) transforms a sequence 𝐹 into an infinite series. seq𝑀( + , 𝐹) ⇝ 2 means "the sum of F(n) from n = M to infinity is 2." Since limits are unique (climuni 14901), by climdm 14903 the "sum of F(n) from n = 1 to infinity" can be expressed as ( ⇝ ‘seq1( + , 𝐹)) (provided the sequence converges) and evaluates to 2 in this example.

Internally, the rec 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 rec is exactly the sequence we want, and we just extract the range (restricted to omega) and throw away the domain.

This definition has its roots in a series of theorems from om2uz0i 13310 through om2uzf1oi 13316, originally proved by Raph Levien for use with df-exp 13426 and later generalized for arbitrary recursive sequences. Definition df-sum 15035 extracts the summation values from partial (finite) and complete (infinite) series. (Contributed by NM, 18-Apr-2005.) (Revised by Mario Carneiro, 4-Sep-2013.)

Assertion

Ref	Expression
df-seq	⊢ seq𝑀( + , 𝐹) = (rec((𝑥 ∈ V, 𝑦 ∈ V ↦ ⟨(𝑥 + 1), (𝑦 + (𝐹‘(𝑥 + 1)))⟩), ⟨𝑀, (𝐹‘𝑀)⟩) “ ω)

Distinct variable groups:   𝑥, + ,𝑦   𝑥,𝐹,𝑦   𝑥,𝑀,𝑦

Detailed syntax breakdown of Definition df-seq

Step	Hyp	Ref	Expression
1		c.pl	. . 3 class +
2		cF	. . 3 class 𝐹
3		cM	. . 3 class 𝑀
4	1, 2, 3	cseq 13364	. 2 class seq𝑀( + , 𝐹)
5		vx	. . . . 5 setvar 𝑥
6		vy	. . . . 5 setvar 𝑦
7		cvv 3441	. . . . 5 class V
8	5	cv 1537	. . . . . . 7 class 𝑥
9		c1 10527	. . . . . . 7 class 1
10		caddc 10529	. . . . . . 7 class +
11	8, 9, 10	co 7135	. . . . . 6 class (𝑥 + 1)
12	6	cv 1537	. . . . . . 7 class 𝑦
13	11, 2	cfv 6324	. . . . . . 7 class (𝐹‘(𝑥 + 1))
14	12, 13, 1	co 7135	. . . . . 6 class (𝑦 + (𝐹‘(𝑥 + 1)))
15	11, 14	cop 4531	. . . . 5 class ⟨(𝑥 + 1), (𝑦 + (𝐹‘(𝑥 + 1)))⟩
16	5, 6, 7, 7, 15	cmpo 7137	. . . 4 class (𝑥 ∈ V, 𝑦 ∈ V ↦ ⟨(𝑥 + 1), (𝑦 + (𝐹‘(𝑥 + 1)))⟩)
17	3, 2	cfv 6324	. . . . 5 class (𝐹‘𝑀)
18	3, 17	cop 4531	. . . 4 class ⟨𝑀, (𝐹‘𝑀)⟩
19	16, 18	crdg 8028	. . 3 class rec((𝑥 ∈ V, 𝑦 ∈ V ↦ ⟨(𝑥 + 1), (𝑦 + (𝐹‘(𝑥 + 1)))⟩), ⟨𝑀, (𝐹‘𝑀)⟩)
20		com 7560	. . 3 class ω
21	19, 20	cima 5522	. 2 class (rec((𝑥 ∈ V, 𝑦 ∈ V ↦ ⟨(𝑥 + 1), (𝑦 + (𝐹‘(𝑥 + 1)))⟩), ⟨𝑀, (𝐹‘𝑀)⟩) “ ω)
22	4, 21	wceq 1538	1 wff seq𝑀( + , 𝐹) = (rec((𝑥 ∈ V, 𝑦 ∈ V ↦ ⟨(𝑥 + 1), (𝑦 + (𝐹‘(𝑥 + 1)))⟩), ⟨𝑀, (𝐹‘𝑀)⟩) “ ω)

This definition is referenced by:  seqex  13366  seqeq1  13367  seqeq2  13368  seqeq3  13369  nfseq  13374  seqval  13375