雙階乘 (double factorial)
正整數 n 的雙階乘寫作 n!!。

用數學表示:





遞歸方式定義:



雙階乘有無限多個,表列前100個如下︰

0!! = 1
1!! = 1
2!! = 2
3!! = 3
4!! = 8
5!! = 15
6!! = 48
7!! = 105
8!! = 384
9!! = 945
10!! = 3840
11!! = 10395
12!! = 46080
13!! = 135135
14!! = 645120
15!! = 2027025
16!! = 10321920
17!! = 34459425
18!! = 185794560
19!! = 654729075
20!! = 3715891200
21!! = 13749310575
22!! = 81749606400
23!! = 316234143225
24!! = 1961990553600
25!! = 7905853580625
26!! = 51011754393600
27!! = 213458046676875
28!! = 1428329123020800
29!! = 6190283353629375
30!! = 42849873690624000
31!! = 191898783962510625
32!! = 1371195958099968000
33!! = 6332659870762850625
34!! = 46620662575398912000
35!! = 221643095476699771875
36!! = 1678343852714360832000
37!! = 8200794532637891559375
38!! = 63777066403145711616000
39!! = 319830986772877770815625
40!! = 2551082656125828464640000
41!! = 13113070457687988603440625
42!! = 107145471557284795514880000
43!! = 563862029680583509947946875
44!! = 4714400748520531002654720000
45!! = 25373791335626257947657609375
46!! = 216862434431944426122117120000
47!! = 1192568192774434123539907640625
48!! = 10409396852733332453861621760000
49!! = 58435841445947272053455474390625
50!! = 520469842636666622693081088000000
51!! = 2980227913743310874726229193921875
52!! = 27064431817106664380040216576000000
53!! = 157952079428395476360490147277859375
54!! = 1461479318123759876522171695104000000
55!! = 8687364368561751199826958100282265625
56!! = 81842841814930553085241614925824000000
57!! = 495179769008019818390136611716089140625
58!! = 4746884825265972078944013665697792000000
59!! = 29215606371473169285018060091249259296875
60!! = 284813089515958324736640819941867520000000
61!! = 1782151988659863326386101665566204817109375
62!! = 17658411549989416133671730836395786240000000
63!! = 112275575285571389562324404930670903477890625
64!! = 1130138339199322632554990773529330319360000000
65!! = 7297912393562140321551086320493608726062890625
66!! = 74589130387155293748629391052935801077760000000
67!! = 488960130368663401543922783473071784646213671875
68!! = 5072060866326559974906798591599634473287680000000
69!! = 33738248995437774706530672059641953140588743359375
70!! = 355044260642859198243475901411974413130137600000000
71!! = 2395415678676082004163677716234578672981800778515625
72!! = 25563186766285862273530264901662157745369907200000000
73!! = 174865344543353986303948473285124243127671456831640625
74!! = 1891675820705153808241239602722999673157373132800000000
75!! = 13114900840751548972796135496384318234575359262373046875
76!! = 143767362373591689426334209806947975159960358092800000000
77!! = 1009847364737869270905302433221592504062302663202724609375
78!! = 11213854265140151775254068364941942062476907931238400000000
79!! = 79777941814291672401518892224505807820921910393015244140625
80!! = 897108341211212142020325469195355364998152634499072000000000
81!! = 6462013286957625464523030270184970433494674741834234775390625
82!! = 73562883979319395645666688474019139929848516028923904000000000
83!! = 536347102817482913555411512425352545980058003572241486357421875
84!! = 6179282254262829234236001831817607754107275346429607936000000000
85!! = 45589503739486047652209978556154966408304930303640526340380859375
86!! = 531418273866603314144296157536314266853225679792946282496000000000
87!! = 3966286825335286145742268134385482077522528936416725791613134765625
88!! = 46764808100261091644698061863195655483083859821779272859648000000000
89!! = 352999527454840466971061863960307904899505075341088595453568994140625
90!! = 4208832729023498248022825567687608993477547383960134557368320000000000
91!! = 32122956998390482494366629620388019345854961856039062186274778466796875
92!! = 387212611070161838818099952227260027399934359324332379277885440000000000
93!! = 2987435000850314871976096554696085799164511452611632783323554397412109375
94!! = 36397985440595212848901395509362442575593829776487243652121231360000000000
95!! = 283806325080779912837729172696128150920628587998105114415737667754150390625
96!! = 3494206602297140433494533968898794487257007658542775390603638210560000000000
97!! = 27529213532835651545259729751524430639300973035816196098326553772152587890625
98!! = 342432247025119762482464328952081859751186750537191988279156544634880000000000
99!! = 2725392139750729502980713245400918633290796330545803413734328823443106201171875
100!! = 34243224702511976248246432895208185975118675053719198827915654463488000000000000

前100項雙階乘亦可參考OEIS的A006882

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