lolcode:

sorry I couldn't resist xD

HAI
CAN HAS STDIO?
I HAS A VAR
I HAS A INT
I HAS A CHEEZBURGER
I HAS A FACTORIALNUM
IM IN YR LOOP
    UP VAR!!1
    TIEMZD INT!![CHEEZBURGER]
    UP FACTORIALNUM!!1
    IZ VAR BIGGER THAN FACTORIALNUM? GTFO
IM OUTTA YR LOOP
U SEEZ INT
KTHXBYE    

This is one of the faster algorithms, up to 170!. It fails inexplicably beyond 170!, and it's relatively slow for small factorials, but for factorials between 80 and 170 it's blazingly fast compared to many algorithms.

curl http://www.google.com/search?q=170!

There's also an online interface, try it out now!

Let me know if you find a bug, or faster implementation for large factorials.

EDIT:

This algorithm is slightly slower, but gives results beyond 170:

curl http://www58.wolframalpha.com/input/?i=171!

It also simplifies them into various other representations.

C++: Template Metaprogramming

Uses the classic enum hack.

template<unsigned int n>
struct factorial {
    enum { result = n * factorial<n - 1>::result };
};

template<>
struct factorial<0> {
    enum { result = 1 };
};

Usage.

const unsigned int x = factorial<4>::result;

Factorial is calculated completely at compile time based on the template parameter n. Therefore, factorial<4>::result is a constant once the compiler has done its work.

Whitespace

   	.
 .
 	.
		.
  	.
   	.
			 .
 .
	 	 .
	  .
   	.
 .
  .
 			 .
		  			 .
 .
	.
.
  	 .
 .
.
	.
 	.
.
.
.

It was hard to get it to show here properly, but now I tried copying it from the preview and it works. You need to input the number and press enter.

I find the following implementations just hilarious:

The Evolution of a Haskell Programmer

Evolution of a Python programmer

Enjoy!

C# Lookup:

Nothing to calculate really, just look it up. To extend it,add another 8 numbers to the table and 64 bit integers are at at their limit. Beyond that, a BigNum class is called for.

public static int Factorial(int f)
{ 
    if (f<0 || f>12)
    {
        throw new ArgumentException("Out of range for integer factorial");
    }
    int [] fact={1,1,2,6,24,120,720,5040,40320,362880,3628800,
                 39916800,479001600};
    return fact[f];
}

Lazy K

Your pure functional programming nightmares come true!

The only Esoteric Turing-complete Programming Language that has:

• A purely functional foundation, core, and libraries---in fact, here's the complete API: S K I
• No lambdas even!
• No numbers or lists needed or allowed
• No explicit recursion but yet, allows recursion
• A simple infinite lazy stream-based I/O mechanism

Here's the Factorial code in all its parenthetical glory:

K(SII(S(K(S(S(KS)(S(K(S(KS)))(S(K(S(KK)))(S(K(S(K(S(K(S(K(S(SI(K(S(K(S(S(KS)K)I))
 (S(S(KS)K)(SII(S(S(KS)K)I))))))))K))))))(S(K(S(K(S(SI(K(S(K(S(SI(K(S(K(S(S(KS)K)I))
 (S(S(KS)K)(SII(S(S(KS)K)I))(S(S(KS)K))(S(SII)I(S(S(KS)K)I))))))))K)))))))
 (S(S(KS)K)(K(S(S(KS)K)))))))))(K(S(K(S(S(KS)K)))K))))(SII))II)

Features:

• No subtraction or conditionals
• Prints all factorials (if you wait long enough)
• Uses a second layer of Church numerals to convert the Nth factorial to N! asterisks followed by a newline
• Uses the Y combinator for recursion

In case you are interested in trying to understand it, here is the Scheme source code to run through the Lazier compiler:

(lazy-def '(fac input)
   '((Y (lambda (f n a) ((lambda (b) ((cons 10) ((b (cons 42)) (f (1+ n) b))))
       (* a n)))) 1 1))

(for suitable definitions of Y, cons, 1, 10, 42, 1+, and *).

EDIT:

Lazy K Factorial in Decimal

(10KB of gibberish or else I would paste it). For example, at the Unix prompt:

    $ echo "4" | ./lazy facdec.lazy
    24
    $ echo "5" | ./lazy facdec.lazy
    120

Rather slow for numbers above, say, 5.

The code is sort of bloated because we have to include library code for all of our own primitives (code written in Hazy, a lambda calculus interpreter and LC-to-Lazy K compiler written in Haskell).

XSLT 1.0

The input file, factorial.xml:

<?xml version="1.0"?>
<?xml-stylesheet href="factorial.xsl" type="text/xsl" ?>
<n>
  20
</n>

The XSLT file, factorial.xsl:

<?xml version="1.0"?>
<xsl:stylesheet version="1.0"                     
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns:msxsl="urn:schemas-microsoft-com:xslt" >
  <xsl:output method="text"/>
  <!-- 0! = 1 -->
  <xsl:template match="text()[. = 0]">
    1
  </xsl:template>
  <!-- n! = (n-1)! * n-->
  <xsl:template match="text()[. > 0]">
    <xsl:variable name="x">
      <xsl:apply-templates select="msxsl:node-set( . - 1 )/text()"/>
    </xsl:variable>
    <xsl:value-of select="$x * ."/>
  </xsl:template>
  <!-- Calculate n! -->
  <xsl:template match="/n">
    <xsl:apply-templates select="text()"/>
  </xsl:template>
</xsl:stylesheet>

Save both files in the same directory and open factorial.xml in IE.

Python: Functional, One-liner

factorial = lambda n: reduce(lambda x,y: x*y, range(1, n+1), 1)

NOTE:

• It supports big integers. Example:

print factorial(100)
93326215443944152681699238856266700490715968264381621468592963895217599993229915\
608941463976156518286253697920827223758251185210916864000000000000000000000000

• It does not work for n < 0.

APL (oddball/one-liner):

×/⍳X

1. ⍳X expands X into an array of the integers 1..X
2. ×/ multiplies every element in the array

Or with the built-in operator:

!X

Source: http://www.webber-labs.com/mpl/lectures/ppt-slides/01.ppt

Perl6

sub factorial ($n) { [*] 1..$n }

I hardly know about Perl6. But I guess this [*] operator is same as Haskell's product.

This code runs on Pugs, and maybe Parrot (I didn't check it.)

Edit

This code also works.

sub postfix:<!> ($n) { [*] 1..$n }

# This function(?) call like below ... It looks like mathematical notation.
say 10!;

x86-64 Assembly: Procedural

You can call this from C (only tested with GCC on linux amd64). Assembly was assembled with nasm.

section .text
    global factorial
; factorial in x86-64 - n is passed in via RDI register
; takes a 64-bit unsigned integer
; returns a 64-bit unsigned integer in RAX register
; C declaration in GCC:
;   extern unsigned long long factorial(unsigned long long n);
factorial:
    enter 0,0
    ; n is placed in rdi by caller
    mov rax, 1 ; factorial = 1
    mov rcx, 2 ; i = 2
loopstart:
    cmp rcx, rdi
    ja loopend
    mul rcx ; factorial *= i
    inc rcx
    jmp loopstart
loopend:
    leave
    ret

Recursively in Inform 7

(it reminds you of COBOL because it's for writing text adventures; proportional font is deliberate):

To decide what number is the factorial of (n - a number):
    if n is zero, decide on one;
    otherwise decide on the factorial of (n minus one) times n.

If you want to actually call this function ("phrase") from a game you need to define an action and grammar rule:

"The factorial game" [this must be the first line of the source]

There is a room. [there has to be at least one!]

Factorialing is an action applying to a number.

Understand "factorial [a number]" as factorialing.

Carry out factorialing:
    Let n be the factorial of the number understood;
    Say "It's [n]".

C#: LINQ

    public static int factorial(int n)
    {
        return (Enumerable.Range(1, n).Aggregate(1, (previous, value) => previous * value));
    }

Erlang: tail recursive

fac(0) -> 1;
fac(N) when N > 0 -> fac(N, 1).

fac(1, R) -> R;
fac(N, R) -> fac(N - 1, R * N).

Haskell:

ones = 1 : ones
integers   = head ones     : zipWith (+) integers   (tail ones)
factorials = head integers : zipWith (*) factorials (tail integers)

Brainf*ck

+++++
>+<[[->>>>+<<<<]>>>>[-<<<<+>>+>>]<<<<>[->>+<<]<>>>[-<[->>+<<]>>[-<<+<+>>>]<]<[-]><<<-]

Written by Michael Reitzenstein.

BASIC: old school

10 HOME
20 INPUT N
30 LET ANS = 1
40 FOR I = 1 TO N
50   ANS = ANS * I
60 NEXT I
70 PRINT ANS

Batch (NT):

@echo off

set n=%1
set result=1

for /l %%i in (%n%, -1, 1) do (
    set /a result=result * %%i
)

echo %result%

Usage: C:>factorial.bat 15

F#: Functional

Straight forward:

let rec fact x = 
    if   x < 0 then failwith "Invalid value."
    elif x = 0 then 1
    else x * fact (x - 1)

Getting fancy:

let fact x = [1 .. x] |> List.fold_left ( * ) 1

Recursive Prolog

fac(0,1).
fac(N,X) :- N1 is N -1, fac(N1, T), X is N * T.

Tail Recursive Prolog

fac(0,N,N).
fac(X,N,T) :- A is N * X, X1 is X - 1, fac(X1,A,T).
fac(N,T) :- fac(N,1,T).

ruby recursive

(factorial=Hash.new{|h,k|k*h[k-1]})[1]=1
    

usage:

factorial[5]
 => 120

Scheme

Here is a simple recursive definition:

(define (factorial x)
  (if (= x 0) 1
      (* x (factorial (- x 1)))))

In Scheme tail-recursive functions use constant stack space. Here is a version of factorial that is tail-recursive:

(define factorial
  (letrec ((fact (lambda (x accum)
                   (if (= x 0) accum
                       (fact (- x 1) (* accum x))))))
    (lambda (x)
      (fact x 1))))

Oddball examples? What about using the gamma function! Since, Gamma n = (n-1)!.

OCaml: Using Gamma

let rec gamma z =
    let pi = 4.0 *. atan 1.0 in
    if z < 0.5 then
        pi /. ((sin (pi*.z)) *. (gamma (1.0 -. z)))
    else
        let consts = [| 0.99999999999980993; 676.5203681218851; -1259.1392167224028;
                        771.32342877765313; -176.61502916214059; 12.507343278686905;
                 -0.13857109526572012; 9.9843695780195716e-6; 1.5056327351493116e-7;
                     |] 
        in
        let z = z -. 1.0 in
        let results = Array.fold_right 
                          (fun x y -> x +. y)
                          (Array.mapi 
                              (fun i x -> if i = 0 then x else x /. (z+.(float i)))
                              consts
                          )
                          0.0
        in
        let x = z +. (float (Array.length consts)) -. 1.5 in
        let final = (sqrt (2.0*.pi)) *. 
                    (x ** (z+.0.5)) *.
                    (exp (-.x)) *. result
        in
        final

let factorial_gamma n = int_of_float (gamma (float (n+1)))

Freshman Haskell programmer

fac n = if n == 0 
           then 1
           else n * fac (n-1)

Sophomore Haskell programmer, at MIT (studied Scheme as a freshman)

fac = (\(n) ->
        (if ((==) n 0)
            then 1
            else ((*) n (fac ((-) n 1)))))

Junior Haskell programmer (beginning Peano player)

fac  0    =  1
fac (n+1) = (n+1) * fac n

Another junior Haskell programmer (read that n+k patterns are “a disgusting part of Haskell” [1] and joined the “Ban n+k patterns”-movement [2])

fac 0 = 1
fac n = n * fac (n-1)

Senior Haskell programmer (voted for Nixon Buchanan Bush — “leans right”)

fac n = foldr (*) 1 [1..n]

Another senior Haskell programmer (voted for McGovern Biafra Nader — “leans left”)

fac n = foldl (*) 1 [1..n]

Yet another senior Haskell programmer (leaned so far right he came back left again!)

-- using foldr to simulate foldl

fac n = foldr (\x g n -> g (x*n)) id [1..n] 1

Memoizing Haskell programmer (takes Ginkgo Biloba daily)

facs = scanl (*) 1 [1..]

fac n = facs !! n

Pointless (ahem) “Points-free” Haskell programmer (studied at Oxford)

fac = foldr (*) 1 . enumFromTo 1

Iterative Haskell programmer (former Pascal programmer)

fac n = result (for init next done)
        where init = (0,1)
              next   (i,m) = (i+1, m * (i+1))
              done   (i,_) = i==n
              result (_,m) = m

for i n d = until d n i

Iterative one-liner Haskell programmer (former APL and C programmer)

fac n = snd (until ((>n) . fst) (\(i,m) -> (i+1, i*m)) (1,1))

Accumulating Haskell programmer (building up to a quick climax)

facAcc a 0 = a
facAcc a n = facAcc (n*a) (n-1)

fac = facAcc 1

Continuation-passing Haskell programmer (raised RABBITS in early years, then moved to New Jersey)

facCps k 0 = k 1
facCps k n = facCps (k . (n *)) (n-1)

fac = facCps id

Boy Scout Haskell programmer (likes tying knots; always “reverent,” he belongs to the Church of the Least Fixed-Point [8])

y f = f (y f)

fac = y (\f n -> if (n==0) then 1 else n * f (n-1))

Combinatory Haskell programmer (eschews variables, if not obfuscation; all this currying’s just a phase, though it seldom hinders)

s f g x = f x (g x)

k x y   = x

b f g x = f (g x)

c f g x = f x g

y f     = f (y f)

cond p f g x = if p x then f x else g x

fac  = y (b (cond ((==) 0) (k 1)) (b (s (*)) (c b pred)))

List-encoding Haskell programmer (prefers to count in unary)

arb = ()    -- "undefined" is also a good RHS, as is "arb" :)

listenc n = replicate n arb
listprj f = length . f . listenc

listprod xs ys = [ i (x,y) | x<-xs, y<-ys ]
                 where i _ = arb

facl []         = listenc  1
facl n@(_:pred) = listprod n (facl pred)

fac = listprj facl

Interpretive Haskell programmer (never “met a language” he didn't like)

-- a dynamically-typed term language

data Term = Occ Var
          | Use Prim
          | Lit Integer
          | App Term Term
          | Abs Var  Term
          | Rec Var  Term

type Var  = String
type Prim = String


-- a domain of values, including functions

data Value = Num  Integer
           | Bool Bool
           | Fun (Value -> Value)

instance Show Value where
  show (Num  n) = show n
  show (Bool b) = show b
  show (Fun  _) = ""

prjFun (Fun f) = f
prjFun  _      = error "bad function value"

prjNum (Num n) = n
prjNum  _      = error "bad numeric value"

prjBool (Bool b) = b
prjBool  _       = error "bad boolean value"

binOp inj f = Fun (\i -> (Fun (\j -> inj (f (prjNum i) (prjNum j)))))


-- environments mapping variables to values

type Env = [(Var, Value)]

getval x env =  case lookup x env of
                  Just v  -> v
                  Nothing -> error ("no value for " ++ x)


-- an environment-based evaluation function

eval env (Occ x) = getval x env
eval env (Use c) = getval c prims
eval env (Lit k) = Num k
eval env (App m n) = prjFun (eval env m) (eval env n)
eval env (Abs x m) = Fun  (\v -> eval ((x,v) : env) m)
eval env (Rec x m) = f where f = eval ((x,f) : env) m


-- a (fixed) "environment" of language primitives

times = binOp Num  (*)

minus = binOp Num  (-)
equal = binOp Bool (==)
cond  = Fun (\b -> Fun (\x -> Fun (\y -> if (prjBool b) then x else y)))

prims = [ ("*", times), ("-", minus), ("==", equal), ("if", cond) ]


-- a term representing factorial and a "wrapper" for evaluation

facTerm = Rec "f" (Abs "n" 
              (App (App (App (Use "if")
                   (App (App (Use "==") (Occ "n")) (Lit 0))) (Lit 1))
                   (App (App (Use "*")  (Occ "n"))
                        (App (Occ "f")  
                             (App (App (Use "-") (Occ "n")) (Lit 1))))))

fac n = prjNum (eval [] (App facTerm (Lit n)))

Static Haskell programmer (he does it with class, he’s got that fundep Jones! After Thomas Hallgren’s “Fun with Functional Dependencies” [7])

-- static Peano constructors and numerals

data Zero
data Succ n

type One   = Succ Zero
type Two   = Succ One
type Three = Succ Two
type Four  = Succ Three


-- dynamic representatives for static Peanos

zero  = undefined :: Zero
one   = undefined :: One
two   = undefined :: Two
three = undefined :: Three
four  = undefined :: Four


-- addition, a la Prolog

class Add a b c | a b -> c where
  add :: a -> b -> c

instance              Add  Zero    b  b
instance Add a b c => Add (Succ a) b (Succ c)


-- multiplication, a la Prolog

class Mul a b c | a b -> c where
  mul :: a -> b -> c

instance                           Mul  Zero    b Zero
instance (Mul a b c, Add b c d) => Mul (Succ a) b d


-- factorial, a la Prolog

class Fac a b | a -> b where
  fac :: a -> b

instance                                Fac  Zero    One
instance (Fac n k, Mul (Succ n) k m) => Fac (Succ n) m

-- try, for "instance" (sorry):
-- 
--     :t fac four

Beginning graduate Haskell programmer (graduate education tends to liberate one from petty concerns about, e.g., the efficiency of hardware-based integers)

-- the natural numbers, a la Peano

data Nat = Zero | Succ Nat


-- iteration and some applications

iter z s  Zero    = z
iter z s (Succ n) = s (iter z s n)

plus n = iter n     Succ
mult n = iter Zero (plus n)


-- primitive recursion

primrec z s  Zero    = z
primrec z s (Succ n) = s n (primrec z s n)


-- two versions of factorial

fac  = snd . iter (one, one) (\(a,b) -> (Succ a, mult a b))
fac' = primrec one (mult . Succ)


-- for convenience and testing (try e.g. "fac five")

int = iter 0 (1+)

instance Show Nat where
  show = show . int

(zero : one : two : three : four : five : _) = iterate Succ Zero

Origamist Haskell programmer (always starts out with the “basic Bird fold”)

-- (curried, list) fold and an application

fold c n []     = n
fold c n (x:xs) = c x (fold c n xs)

prod = fold (*) 1


-- (curried, boolean-based, list) unfold and an application

unfold p f g x = 
  if p x 
     then [] 
     else f x : unfold p f g (g x)

downfrom = unfold (==0) id pred


-- hylomorphisms, as-is or "unfolded" (ouch! sorry ...)

refold  c n p f g   = fold c n . unfold p f g

refold' c n p f g x = 
  if p x 
     then n 
     else c (f x) (refold' c n p f g (g x))


-- several versions of factorial, all (extensionally) equivalent

fac   = prod . downfrom
fac'  = refold  (*) 1 (==0) id pred
fac'' = refold' (*) 1 (==0) id pred

Cartesianally-inclined Haskell programmer (prefers Greek food, avoids the spicy Indian stuff; inspired by Lex Augusteijn’s “Sorting Morphisms” [3])

-- (product-based, list) catamorphisms and an application

cata (n,c) []     = n
cata (n,c) (x:xs) = c (x, cata (n,c) xs)

mult = uncurry (*)
prod = cata (1, mult)


-- (co-product-based, list) anamorphisms and an application

ana f = either (const []) (cons . pair (id, ana f)) . f

cons = uncurry (:)

downfrom = ana uncount

uncount 0 = Left  ()
uncount n = Right (n, n-1)


-- two variations on list hylomorphisms

hylo  f  g    = cata g . ana f

hylo' f (n,c) = either (const n) (c . pair (id, hylo' f (c,n))) . f

pair (f,g) (x,y) = (f x, g y)


-- several versions of factorial, all (extensionally) equivalent

fac   = prod . downfrom
fac'  = hylo  uncount (1, mult)
fac'' = hylo' uncount (1, mult)

Ph.D. Haskell programmer (ate so many bananas that his eyes bugged out, now he needs new lenses!)

-- explicit type recursion based on functors

newtype Mu f = Mu (f (Mu f))  deriving Show

in      x  = Mu x
out (Mu x) = x


-- cata- and ana-morphisms, now for *arbitrary* (regular) base functors

cata phi = phi . fmap (cata phi) . out
ana  psi = in  . fmap (ana  psi) . psi


-- base functor and data type for natural numbers,
-- using a curried elimination operator

data N b = Zero | Succ b  deriving Show

instance Functor N where
  fmap f = nelim Zero (Succ . f)

nelim z s  Zero    = z
nelim z s (Succ n) = s n

type Nat = Mu N


-- conversion to internal numbers, conveniences and applications

int = cata (nelim 0 (1+))

instance Show Nat where
  show = show . int

zero = in   Zero
suck = in . Succ       -- pardon my "French" (Prelude conflict)

plus n = cata (nelim n     suck   )
mult n = cata (nelim zero (plus n))


-- base functor and data type for lists

data L a b = Nil | Cons a b  deriving Show

instance Functor (L a) where
  fmap f = lelim Nil (\a b -> Cons a (f b))

lelim n c  Nil       = n
lelim n c (Cons a b) = c a b

type List a = Mu (L a)


-- conversion to internal lists, conveniences and applications

list = cata (lelim [] (:))

instance Show a => Show (List a) where
  show = show . list

prod = cata (lelim (suck zero) mult)

upto = ana (nelim Nil (diag (Cons . suck)) . out)

diag f x = f x x

fac = prod . upto

Post-doc Haskell programmer (from Uustalu, Vene and Pardo’s “Recursion Schemes from Comonads” [4])

-- explicit type recursion with functors and catamorphisms

newtype Mu f = In (f (Mu f))

unIn (In x) = x

cata phi = phi . fmap (cata phi) . unIn


-- base functor and data type for natural numbers,
-- using locally-defined "eliminators"

data N c = Z | S c

instance Functor N where
  fmap g  Z    = Z
  fmap g (S x) = S (g x)

type Nat = Mu N

zero   = In  Z
suck n = In (S n)

add m = cata phi where
  phi  Z    = m
  phi (S f) = suck f

mult m = cata phi where
  phi  Z    = zero
  phi (S f) = add m f


-- explicit products and their functorial action

data Prod e c = Pair c e

outl (Pair x y) = x
outr (Pair x y) = y

fork f g x = Pair (f x) (g x)

instance Functor (Prod e) where
  fmap g = fork (g . outl) outr


-- comonads, the categorical "opposite" of monads

class Functor n => Comonad n where
  extr :: n a -> a
  dupl :: n a -> n (n a)

instance Comonad (Prod e) where
  extr = outl
  dupl = fork id outr


-- generalized catamorphisms, zygomorphisms and paramorphisms

gcata :: (Functor f, Comonad n) =>
           (forall a. f (n a) -> n (f a))
             -> (f (n c) -> c) -> Mu f -> c

gcata dist phi = extr . cata (fmap phi . dist . fmap dupl)

zygo chi = gcata (fork (fmap outl) (chi . fmap outr))

para :: Functor f => (f (Prod (Mu f) c) -> c) -> Mu f -> c
para = zygo In


-- factorial, the *hard* way!

fac = para phi where
  phi  Z             = suck zero
  phi (S (Pair f n)) = mult f (suck n)


-- for convenience and testing

int = cata phi where
  phi  Z    = 0
  phi (S f) = 1 + f

instance Show (Mu N) where
  show = show . int

Tenured professor (teaching Haskell to freshmen)

fac n = product [1..n]

D Templates: Functional

template factorial(int n : 1)
{
  const factorial = 1;
}

template factorial(int n)
{
  const factorial =
     n * factorial!(n-1);
}

or

template factorial(int n)
{
  static if(n == 1)
    const factorial = 1;
  else 
    const factorial =
       n * factorial!(n-1);
}

Used like this:

factorial!(5)

Java 1.6: recursive, memoized (for subsequent calls)

private static Map<BigInteger, BigInteger> _results = new HashMap()

public static BigInteger factorial(BigInteger n){
    if (0 >= n.compareTo(BigInteger.ONE))
       return BigInteger.ONE.max(n);
    if (_results.containsKey(n))
       return _results.get(n);
    BigInteger result = factorial(n.subtract(BigInteger.ONE)).multiply(n);
    _results.put(n, result);
    return result;
}

PowerShell

function factorial( [int] $n ) 
{ 
    $result = 1; 

    if ( $n -gt 1 ) 
    { 
        $result = $n * ( factorial ( $n - 1 ) ) 
    } 

    $result 
}

Here's a one-liner:

$n..1 | % {$result = 1}{$result *= $_}{$result}

Bash: Recursive

In bash and recursive, but with the added advantage that it deals with each iteration in a new process. The max it can calculate is !20 before overflowing, but you can still run it for big numbers if you don't care about the answer and want your system to fall over ;)

#!/bin/bash
echo $(($1 * `( [[ $1 -gt 1 ]] && ./$0 $(($1 - 1)) ) || echo 1`));