sgnsgn - Mathbox for Thierry Arnoux

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Theorem sgnsgn 32916

Description: Signum is idempotent. (Contributed by Thierry Arnoux, 2-Oct-2018.)

**Assertion**
Ref	Expression
sgnsgn	⊢ (𝐴 ∈ ℝ^* → (sgn‘(sgn‘𝐴)) = (sgn‘𝐴))

**Proof of Theorem sgnsgn**
Step	Hyp	Ref	Expression
1		id 22	. 2 ⊢ (𝐴 ∈ ℝ^* → 𝐴 ∈ ℝ^*)
2		fveq2 6842	. . 3 ⊢ ((sgn‘𝐴) = 0 → (sgn‘(sgn‘𝐴)) = (sgn‘0))
3		id 22	. . 3 ⊢ ((sgn‘𝐴) = 0 → (sgn‘𝐴) = 0)
4	2, 3	eqeq12d 2753	. 2 ⊢ ((sgn‘𝐴) = 0 → ((sgn‘(sgn‘𝐴)) = (sgn‘𝐴) ↔ (sgn‘0) = 0))
5		fveq2 6842	. . 3 ⊢ ((sgn‘𝐴) = 1 → (sgn‘(sgn‘𝐴)) = (sgn‘1))
6		id 22	. . 3 ⊢ ((sgn‘𝐴) = 1 → (sgn‘𝐴) = 1)
7	5, 6	eqeq12d 2753	. 2 ⊢ ((sgn‘𝐴) = 1 → ((sgn‘(sgn‘𝐴)) = (sgn‘𝐴) ↔ (sgn‘1) = 1))
8		fveq2 6842	. . 3 ⊢ ((sgn‘𝐴) = -1 → (sgn‘(sgn‘𝐴)) = (sgn‘-1))
9		id 22	. . 3 ⊢ ((sgn‘𝐴) = -1 → (sgn‘𝐴) = -1)
10	8, 9	eqeq12d 2753	. 2 ⊢ ((sgn‘𝐴) = -1 → ((sgn‘(sgn‘𝐴)) = (sgn‘𝐴) ↔ (sgn‘-1) = -1))
11		sgn0 15053	. . 3 ⊢ (sgn‘0) = 0
12	11	a1i 11	. 2 ⊢ ((𝐴 ∈ ℝ^* ∧ 𝐴 = 0) → (sgn‘0) = 0)
13		sgn1 15056	. . 3 ⊢ (sgn‘1) = 1
14	13	a1i 11	. 2 ⊢ ((𝐴 ∈ ℝ^* ∧ 0 < 𝐴) → (sgn‘1) = 1)
15		neg1rr 12147	. . . . 5 ⊢ -1 ∈ ℝ
16	15	rexri 11205	. . . 4 ⊢ -1 ∈ ℝ^*
17		neg1lt0 12149	. . . 4 ⊢ -1 < 0
18		sgnn 15058	. . . 4 ⊢ ((-1 ∈ ℝ^* ∧ -1 < 0) → (sgn‘-1) = -1)
19	16, 17, 18	mp2an 693	. . 3 ⊢ (sgn‘-1) = -1
20	19	a1i 11	. 2 ⊢ ((𝐴 ∈ ℝ^* ∧ 𝐴 < 0) → (sgn‘-1) = -1)
21	1, 4, 7, 10, 12, 14, 20	sgn3da 32909	1 ⊢ (𝐴 ∈ ℝ^* → (sgn‘(sgn‘𝐴)) = (sgn‘𝐴))

Colors of variables: wff setvar class

Syntax hints: → wi 4 ∧ wa 395 = wceq 1542 ∈ wcel 2114 class class class wbr 5086 ‘cfv 6500 0cc0 11040 1c1 11041 ℝ^*cxr 11180 < clt 11181 -cneg 11380 sgncsgn 15050

This theorem was proved from axioms: ax-mp 5 ax-1 6 ax-2 7 ax-3 8 ax-gen 1797 ax-4 1811 ax-5 1912 ax-6 1969 ax-7 2010 ax-8 2116 ax-9 2124 ax-10 2147 ax-11 2163 ax-12 2185 ax-ext 2709 ax-sep 5232 ax-nul 5242 ax-pow 5308 ax-pr 5376 ax-un 7691 ax-cnex 11096 ax-resscn 11097 ax-1cn 11098 ax-icn 11099 ax-addcl 11100 ax-addrcl 11101 ax-mulcl 11102 ax-mulrcl 11103 ax-mulcom 11104 ax-addass 11105 ax-mulass 11106 ax-distr 11107 ax-i2m1 11108 ax-1ne0 11109 ax-1rid 11110 ax-rnegex 11111 ax-rrecex 11112 ax-cnre 11113 ax-pre-lttri 11114 ax-pre-lttrn 11115 ax-pre-ltadd 11116 ax-pre-mulgt0 11117

This theorem depends on definitions: df-bi 207 df-an 396 df-or 849 df-3or 1088 df-3an 1089 df-tru 1545 df-fal 1555 df-ex 1782 df-nf 1786 df-sb 2069 df-mo 2540 df-eu 2570 df-clab 2716 df-cleq 2729 df-clel 2812 df-nfc 2886 df-ne 2934 df-nel 3038 df-ral 3053 df-rex 3063 df-reu 3344 df-rab 3391 df-v 3432 df-sbc 3730 df-csb 3839 df-dif 3893 df-un 3895 df-in 3897 df-ss 3907 df-nul 4275 df-if 4468 df-pw 4544 df-sn 4569 df-pr 4571 df-op 4575 df-uni 4852 df-br 5087 df-opab 5149 df-mpt 5168 df-id 5527 df-po 5540 df-so 5541 df-xp 5638 df-rel 5639 df-cnv 5640 df-co 5641 df-dm 5642 df-rn 5643 df-res 5644 df-ima 5645 df-iota 6456 df-fun 6502 df-fn 6503 df-f 6504 df-f1 6505 df-fo 6506 df-f1o 6507 df-fv 6508 df-riota 7326 df-ov 7372 df-oprab 7373 df-mpo 7374 df-er 8645 df-en 8896 df-dom 8897 df-sdom 8898 df-pnf 11183 df-mnf 11184 df-xr 11185 df-ltxr 11186 df-le 11187 df-sub 11381 df-neg 11382 df-sgn 15051

This theorem is referenced by: signsvfn 34728 signsvfpn 34731 signsvfnn 34732

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