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Theorem fac0 14233

Description: The factorial of 0. (Contributed by NM, 2-Dec-2004.) (Revised by Mario Carneiro, 13-Jul-2013.)

**Assertion**
Ref	Expression
fac0	⊢ (!‘0) = 1

**Proof of Theorem fac0**
Step	Hyp	Ref	Expression
1		c0ex 11134	. . . 4 ⊢ 0 ∈ V
2	1	a1i 11	. . 3 ⊢ (⊤ → 0 ∈ V)
3		1ex 11136	. . . 4 ⊢ 1 ∈ V
4	3	a1i 11	. . 3 ⊢ (⊤ → 1 ∈ V)
5		df-fac 14231	. . . 4 ⊢ ! = ({⟨0, 1⟩} ∪ seq1( · , I ))
6		nnuz 12822	. . . . . . . 8 ⊢ ℕ = (ℤ_≥‘1)
7		dfn2 12445	. . . . . . . 8 ⊢ ℕ = (ℕ₀ ∖ {0})
8	6, 7	eqtr3i 2766	. . . . . . 7 ⊢ (ℤ_≥‘1) = (ℕ₀ ∖ {0})
9	8	reseq2i 5934	. . . . . 6 ⊢ (seq1( · , I ) ↾ (ℤ_≥‘1)) = (seq1( · , I ) ↾ (ℕ₀ ∖ {0}))
10		1z 12552	. . . . . . 7 ⊢ 1 ∈ ℤ
11		seqfn 13970	. . . . . . 7 ⊢ (1 ∈ ℤ → seq1( · , I ) Fn (ℤ_≥‘1))
12		fnresdm 6607	. . . . . . 7 ⊢ (seq1( · , I ) Fn (ℤ_≥‘1) → (seq1( · , I ) ↾ (ℤ_≥‘1)) = seq1( · , I ))
13	10, 11, 12	mp2b 10	. . . . . 6 ⊢ (seq1( · , I ) ↾ (ℤ_≥‘1)) = seq1( · , I )
14	9, 13	eqtr3i 2766	. . . . 5 ⊢ (seq1( · , I ) ↾ (ℕ₀ ∖ {0})) = seq1( · , I )
15	14	uneq2i 4097	. . . 4 ⊢ ({⟨0, 1⟩} ∪ (seq1( · , I ) ↾ (ℕ₀ ∖ {0}))) = ({⟨0, 1⟩} ∪ seq1( · , I ))
16	5, 15	eqtr4i 2767	. . 3 ⊢ ! = ({⟨0, 1⟩} ∪ (seq1( · , I ) ↾ (ℕ₀ ∖ {0})))
17	2, 4, 16	fvsnun1 7129	. 2 ⊢ (⊤ → (!‘0) = 1)
18	17	mptru 1555	1 ⊢ (!‘0) = 1

Colors of variables: wff setvar class

Syntax hints: = wceq 1548 ⊤wtru 1549 ∈ wcel 2121 Vcvv 3433 ∖ cdif 3881 ∪ cun 3882 {csn 4557 ⟨cop 4563 I cid 5514 ↾ cres 5622 Fn wfn 6483 ‘cfv 6488 0cc0 11034 1c1 11035 · cmul 11039 ℕcn 12169 ℕ₀cn0 12432 ℤcz 12519 ℤ_≥cuz 12783 seqcseq 13958 !cfa 14230

This theorem was proved from axioms: ax-mp 5 ax-1 6 ax-2 7 ax-3 8 ax-gen 1803 ax-4 1817 ax-5 1918 ax-6 1975 ax-7 2016 ax-8 2123 ax-9 2131 ax-10 2154 ax-11 2170 ax-12 2191 ax-ext 2713 ax-sep 5220 ax-nul 5230 ax-pow 5296 ax-pr 5364 ax-un 7681 ax-cnex 11090 ax-resscn 11091 ax-1cn 11092 ax-icn 11093 ax-addcl 11094 ax-addrcl 11095 ax-mulcl 11096 ax-mulrcl 11097 ax-mulcom 11098 ax-addass 11099 ax-mulass 11100 ax-distr 11101 ax-i2m1 11102 ax-1ne0 11103 ax-1rid 11104 ax-rnegex 11105 ax-rrecex 11106 ax-cnre 11107 ax-pre-lttri 11108 ax-pre-lttrn 11109 ax-pre-ltadd 11110 ax-pre-mulgt0 11111

This theorem depends on definitions: df-bi 209 df-an 398 df-or 855 df-3or 1094 df-3an 1095 df-tru 1551 df-fal 1561 df-ex 1788 df-nf 1792 df-sb 2075 df-mo 2545 df-eu 2575 df-clab 2720 df-cleq 2733 df-clel 2816 df-nfc 2890 df-ne 2937 df-nel 3041 df-ral 3056 df-rex 3066 df-reu 3347 df-rab 3394 df-v 3435 df-sbc 3725 df-csb 3833 df-dif 3887 df-un 3889 df-in 3891 df-ss 3901 df-pss 3904 df-nul 4264 df-if 4457 df-pw 4533 df-sn 4558 df-pr 4560 df-op 4564 df-uni 4841 df-iun 4925 df-br 5075 df-opab 5137 df-mpt 5156 df-tr 5182 df-id 5515 df-eprel 5520 df-po 5528 df-so 5529 df-fr 5573 df-we 5575 df-xp 5626 df-rel 5627 df-cnv 5628 df-co 5629 df-dm 5630 df-rn 5631 df-res 5632 df-ima 5633 df-pred 6255 df-ord 6316 df-on 6317 df-lim 6318 df-suc 6319 df-iota 6444 df-fun 6490 df-fn 6491 df-f 6492 df-f1 6493 df-fo 6494 df-f1o 6495 df-fv 6496 df-riota 7316 df-ov 7362 df-oprab 7363 df-mpo 7364 df-om 7810 df-2nd 7934 df-frecs 8224 df-wrecs 8255 df-recs 8304 df-rdg 8343 df-er 8637 df-en 8888 df-dom 8889 df-sdom 8890 df-pnf 11177 df-mnf 11178 df-xr 11179 df-ltxr 11180 df-le 11181 df-sub 11375 df-neg 11376 df-nn 12170 df-n0 12433 df-z 12520 df-uz 12784 df-seq 13959 df-fac 14231

This theorem is referenced by: facp1 14235 faccl 14240 facwordi 14246 faclbnd 14247 faclbnd4lem3 14252 facubnd 14257 bcn0 14267 bcval5 14275 hashf1 14414 fprodfac 15933 fallfacfac 16005 ef0lem 16038 ege2le3 16050 eft0val 16074 prmfac1 16685 pcfac 16865 tayl0 26348 logfac 26586 advlogexp 26640 facgam 27050 logexprlim 27209 subfacval2 35428 faclim 35987 bccn0 44800 mccl 46055 dvnxpaek 46397 dvnprodlem3 46403 etransclem14 46703 etransclem24 46713 etransclem25 46714 etransclem35 46724

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