Definition df-exp 10631

Description: Define exponentiation to nonnegative integer powers. For example, (5↑2) = 25 (see ex-exp 15373).

This definition is not meant to be used directly; instead, exp0 10635 and expp1 10638 provide the standard recursive definition. The up-arrow notation is used by Donald Knuth for iterated exponentiation (Science 194, 1235-1242, 1976) and is convenient for us since we don't have superscripts.

10-Jun-2005: The definition was extended to include zero exponents, so that 0↑0 = 1 per the convention of Definition 10-4.1 of [Gleason] p. 134 (see 0exp0e1 10636).

4-Jun-2014: The definition was extended to include negative integer exponents. For example, (-3↑-2) = (1 / 9) (ex-exp 15373). The case 𝑥 = 0, 𝑦 < 0 gives the value (1 / 0), so we will avoid this case in our theorems. (Contributed by Raph Levien, 20-May-2004.) (Revised by NM, 15-Oct-2004.)

Assertion

Ref	Expression
df-exp	⊢ ↑ = (𝑥 ∈ ℂ, 𝑦 ∈ ℤ ↦ if(𝑦 = 0, 1, if(0 < 𝑦, (seq1( · , (ℕ × {𝑥}))‘𝑦), (1 / (seq1( · , (ℕ × {𝑥}))‘-𝑦)))))

Distinct variable group:   𝑥,𝑦

Detailed syntax breakdown of Definition df-exp

Step	Hyp	Ref	Expression
1		cexp 10630	. 2 class ↑
2		vx	. . 3 setvar 𝑥
3		vy	. . 3 setvar 𝑦
4		cc 7877	. . 3 class ℂ
5		cz 9326	. . 3 class ℤ
6	3	cv 1363	. . . . 5 class 𝑦
7		cc0 7879	. . . . 5 class 0
8	6, 7	wceq 1364	. . . 4 wff 𝑦 = 0
9		c1 7880	. . . 4 class 1
10		clt 8061	. . . . . 6 class <
11	7, 6, 10	wbr 4033	. . . . 5 wff 0 < 𝑦
12		cmul 7884	. . . . . . 7 class ·
13		cn 8990	. . . . . . . 8 class ℕ
14	2	cv 1363	. . . . . . . . 9 class 𝑥
15	14	csn 3622	. . . . . . . 8 class {𝑥}
16	13, 15	cxp 4661	. . . . . . 7 class (ℕ × {𝑥})
17	12, 16, 9	cseq 10539	. . . . . 6 class seq1( · , (ℕ × {𝑥}))
18	6, 17	cfv 5258	. . . . 5 class (seq1( · , (ℕ × {𝑥}))‘𝑦)
19	6	cneg 8198	. . . . . . 7 class -𝑦
20	19, 17	cfv 5258	. . . . . 6 class (seq1( · , (ℕ × {𝑥}))‘-𝑦)
21		cdiv 8699	. . . . . 6 class /
22	9, 20, 21	co 5922	. . . . 5 class (1 / (seq1( · , (ℕ × {𝑥}))‘-𝑦))
23	11, 18, 22	cif 3561	. . . 4 class if(0 < 𝑦, (seq1( · , (ℕ × {𝑥}))‘𝑦), (1 / (seq1( · , (ℕ × {𝑥}))‘-𝑦)))
24	8, 9, 23	cif 3561	. . 3 class if(𝑦 = 0, 1, if(0 < 𝑦, (seq1( · , (ℕ × {𝑥}))‘𝑦), (1 / (seq1( · , (ℕ × {𝑥}))‘-𝑦))))
25	2, 3, 4, 5, 24	cmpo 5924	. 2 class (𝑥 ∈ ℂ, 𝑦 ∈ ℤ ↦ if(𝑦 = 0, 1, if(0 < 𝑦, (seq1( · , (ℕ × {𝑥}))‘𝑦), (1 / (seq1( · , (ℕ × {𝑥}))‘-𝑦)))))
26	1, 25	wceq 1364	1 wff ↑ = (𝑥 ∈ ℂ, 𝑦 ∈ ℤ ↦ if(𝑦 = 0, 1, if(0 < 𝑦, (seq1( · , (ℕ × {𝑥}))‘𝑦), (1 / (seq1( · , (ℕ × {𝑥}))‘-𝑦)))))

This definition is referenced by:  exp3val  10633