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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