# Difference between revisions of "Factorial"

The factorial is an important function in combinatorics and analysis, used to determine the number of ways to arrange objects.

## Definition

The factorial is defined for positive integers as $n!=n \cdot (n-1) \cdots 2 \cdot 1 = \prod_{i=1}^n i$. Alternatively, a recursive definition for the factorial is $n!=n \cdot (n-1)!$.

## Examples

• $0! = 1$
• $1! = 1$
• $2! = 2$
• $3! = 6$
• $4! = 24$
• $5! = 120$
• $6! = 720$
• $7! = 5040$
• $31! = 8222838654177922817725562880000000$
• $32! = 263130836933693530167218012160000000$
• $33! = 8683317618811886495518194401280000000$
• $34! = 295232799039604140847618609643520000000$
• $35! = 10333147966386144929666651337523200000000$
• $36! = 371993326789901217467999448150835200000000$
• $37! = 13763753091226345046315979581580902400000000$
• $38! = 523022617466601111760007224100074291200000000$
• $39! = 20397882081197443358640281739902897356800000000$
• $40! = 815915283247897734345611269596115894272000000000$
• $41! = 33452526613163807108170062053440751665152000000000$
• $42! = 1405006117752879898543142606244511569936384000000000$
• $43! = 60415263063373835637355132068513997507264512000000000$
• $44! = 2658271574788448768043625811014615890319638528000000000$
• $45! = 119622220865480194561963161495657715064383733760000000000$
• $46! = 5502622159812088949850305428800254892961651752960000000000$
• $47! = 258623241511168180642964355153611979969197632389120000000000$
• $48! = 12413915592536072670862289047373375038521486354677760000000000$
• $49! = 608281864034267560872252163321295376887552831379210240000000000$
• $50! = 30414093201713378043612608166064768844377641568960512000000000000$
• $51! = 1551118753287382280224243016469303211063259720016986112000000000000$
• $52! = 80658175170943878571660636856403766975289505440883277824000000000000$
• $53! = 4274883284060025564298013753389399649690343788366813724672000000000000$
• $54! = 230843697339241380472092742683027581083278564571807941132288000000000000$
• $55! = 12696403353658275925965100847566516959580321051449436762275840000000000000$
• $56! = 710998587804863451854045647463724949736497978881168458687447040000000000000$
• $57! = 40526919504877216755680601905432322134980384796226602145184481280000000000000$
• $58! = 2350561331282878571829474910515074683828862318181142924420699914240000000000000$
• $59! = 138683118545689835737939019720389406345902876772687432540821294940160000000000000$
• $60! = 8320987112741390144276341183223364380754172606361245952449277696409600000000000000$ (Note: this number is 82 digits long with 14 terminal zeroes!)
• $1000!$ is 2568 digits long and has as much as 249 terminal zeroes!
• $10000!$ is 38660 digits long and has 2499 terminal zeroes!
• $100000!$ is 456574 digits long and has 24999 terminal zeroes!
• $200000!$ is 973751 digits long and has 49998 terminal zeroes!

By convention and rules of an empty product, $0!$ is given the value $1$.

The gamma function is a generalization of the factorial to values other than nonnegative integers.

## Prime Factorization

Main article: Prime factorization

Since $n!$ is the product of all positive integers not exceeding $n$, it is clear that it is divisible by all primes $p\le n$, and not divisible by any prime $p>n$. But what is the power of a prime $p\le n$ in the prime factorization of $n!$? We can find it as the sum of powers of $p$ in all the factors $1,2,\dots, n$; but rather than counting the power of $p$ in each factor, we shall count the number of factors divisible by a given power of $p$. Among the numbers $1,2,\dots,n$, exactly $\left\lfloor\frac n{p^k}\right\rfloor$ are divisible by $p^k$ (here $\lfloor\cdot\rfloor$ is the floor function). The ones divisible by $p$ give one power of $p$. The ones divisible by $p^2$ give another power of $p$. Those divisible by $p^3$ give yet another power of $p$. Continuing in this manner gives $\left\lfloor\frac n{p}\right\rfloor+ \left\lfloor\frac n{p^2}\right\rfloor+ \left\lfloor\frac n{p^3}\right\rfloor+\dots$

for the power of $p$ in the prime factorization of $n!$. The series is formally infinite, but the terms converge to $0$ rapidly, as it is the reciprocal of an exponential function. For example, the power of $7$ in $100!$ is just $\left\lfloor\frac {100}{7}\right\rfloor+ \left\lfloor\frac {100}{49}\right\rfloor=14+2=16$ ( $7^3=343$ is already greater than $100$).

## Uses

The factorial is used in the definitions of combinations and permutations, as $n!$ is the number of ways to order $n$ distinct objects.

## Problems

### Introductory

• Find the units digit of the sum $$\sum_{i=1}^{100}(i!)^{2}$$ $\mathrm{(A)}\,0\quad\mathrm{(B)}\,1\quad\mathrm{(C)}\,3\quad\mathrm{(D)}\,5\quad\mathrm{(E)}\,7\quad\mathrm{(F)}\,9$ (Source)

### Intermediate

• $\frac{((3!)!)!}{3!}=k*n!$, where $k$ and $n$ are positive integers and $n$ is as large as possible. Find the value of $k+n$.

(Source)

• Let $P$ be the product of the first $100$ positive odd integers. Find the largest integer $k$ such that $P$ is divisible by $3^k .$

(Source)

• Let $p_n (k)$ be the number of permutations of the set $\{ 1, \ldots , n \} , \; n \ge 1$, which have exactly $k$ fixed points. Prove that $\sum_{k=0}^{n} k \cdot p_n (k) = n!$.
On that link, you can calculate factorials from $0!$ to as much as $200000!$