4

== Number ==
$4$ (four) is:
:$2^2$
:The only number which equals the number of letters in its name (four) when written in the English language
:The only composite number $n$ such that $n \nmid \paren {n - 1}!$
:The only number $n$ such that the alternating group $A_n$ is not simple.
=== $1$st Term ===
:The $1$st semiprime:
::$4 = 2 \times 2$
:The $1$st power of $4$ after the zeroth $1$:
::$4 = 4^1$
:The $1$st Fermat pseudoprime to base $5$:
::$5^4 \equiv 5 \pmod 4$
:The $1$st $n$ such that $n! + 1$ is square: see Brocard's Problem
:The $1$st positive integer $n$ such that $n - 2^k$ is prime for all $k$.
:The $1$st (with $121$) of the $2$ square numbers which are $4$ less than a cube:
::$4 = 2^2 = 2^3 - 4$
:The $1$st in the sequence formed by adding the squares of the first $n$ primes:
::$\ds 4 = \sum_{i \mathop = 1}^1 {p_i}^2 = 2^2$
:The $1$st Smith number:
::$4 = 2 + 2$
:The smallest positive integer which can be expressed as the sum of $2$ distinct lucky numbers in $1$ different way:
::$4 = 1 + 3$
:The number of primes with no more than $1$ digit:
::$2$, $3$, $5$, $7$
=== $2$nd Term ===
:The $2$nd square number:
::$4 = 2 \times 2 = 2^2 = 1 + 3$
:The first square of a prime number:
::$4 = 2^2$
:The $2$nd square number after $1$ to be the divisor sum value of some (strictly) positive integer:
::$4 = \map {\sigma_1} 3$
:The $2$nd tetrahedral number after $1$:
::$4 = 1 + 3 = \dfrac {2 \paren {2 + 1} \paren {2 + 2} } 6$
:The $2$nd after $1$ of the $3$ tetrahedral numbers which are also square
:The $2$nd trimorphic number after $1$:
::$4^3 = 6 \mathbf 4$
:The $2$nd power of $2$ after $(1)$, $2$:
::$4 = 2^2$
:The $2$nd powerful number after $1$
:The $2$nd integer $m$ after $3$ such that $m! - 1$ (its factorial minus $1$) is prime:
::$4! - 1 = 24 - 1 = 23$
:The $2$nd integer $n$ after $3$ such that $\ds m = \sum_{k \mathop = 0}^{n - 1} \paren {-1}^k \paren {n - k}! = n! - \paren {n - 1}! + \paren {n - 2}! - \paren {n - 3}! + \cdots \pm 1$ is prime:
::$4! - 3! + 2! - 1! = 17$
:The $2$nd even number after $2$ which cannot be expressed as the sum of $2$ composite odd numbers.
:The $2$nd square after $1$ which has no more than $2$ distinct digits and does not end in $0$:
::$4 = 2^2$
:The $2$nd even integer after $2$ that cannot be expressed as the sum of $2$ prime numbers which are each one of a pair of twin primes.
:The $2$nd power of $2$ after $1$ which is the sum of distinct powers of $3$:
::$4 = 2^2 = 3^0 + 3^1$
:The $2$nd of $35$ integers less than $91$ to which $91$ itself is a Fermat pseudoprime:
::$3$, $4$, $\ldots$
:The $2$nd integer $n$ after $3$ for which the Ramanujan-Nagell equation $x^2 + 7 = 2^n$ has an integral solution:
::$3^2 + 7 = 16 = 2^4$
:The number of different binary operations with an identity element that can be applied to a set with $2$ elements
:The total number of permutations of $r$ objects from a set of $2$ objects, where $1 \le r \le 2$
=== $3$rd Term ===
:The $3$rd Lucas number after $(2)$, $1$, $3$:
::$4 = 1 + 3$
:The $3$rd highly composite number after $1$, $2$:
::$\map {\sigma_0} 4 = 3$
:The $3$rd superabundant number after $1$, $2$:
::$\dfrac {\map {\sigma_1} 4} 4 = \dfrac 7 4 = 1 \cdotp 75$
:The $3$rd almost perfect number after $1$, $2$:
::$\map {\sigma_1} 4 = 7 = 8 - 1$
:The $3$rd after $1$, $2$ of the $24$ positive integers which cannot be expressed as the sum of distinct non-pythagorean primes.
:The $3$rd of the $5$ known powers of $2$ whose digits are also all powers of $2$:
::$1$, $2$, $4$, $\ldots$
:The $3$rd positive integer after $2$, $3$ which cannot be expressed as the sum of distinct pentagonal numbers
=== $4$th Term ===
:The $4$th Ulam number after $1$, $2$, $3$:
::$4 = 1 + 3$
:The $4$th highly abundant number after $1$, $2$, $3$:
::$\map {\sigma_1} 4 = 7$
:The $4$th (strictly) positive integer after $1$, $2$, $3$ which cannot be expressed as the sum of exactly $5$ non-zero squares.
:The $4$th integer after $0$, $1$, $2$ which is palindromic in both decimal and ternary:
::$4_{10} = 11_3$
:The $4$th of the trivial $1$-digit pluperfect digital invariants after $1$, $2$, $3$:
::$4^1 = 4$
:The $4$th positive integer after $1$, $2$, $3$ such that all smaller positive integers coprime to it are prime
:The $4$th of the (trivial $1$-digit) Zuckerman numbers after $1$, $2$, $3$:
::$4 = 1 \times 4$
:The $4$th of the (trivial $1$-digit) harshad numbers after $1$, $2$, $3$:
::$4 = 1 \times 4$
:The $4$th (strictly) positive integer after $1$, $2$, $3$ which cannot be expressed as the sum of distinct primes of the form $6 n - 1$
:The $4$th after $1$, $2$, $3$ of $21$ integers which can be represented as the sum of two primes in the maximum number of ways
=== $5$th Term ===
:The $5$th integer $n$ after $0$, $1$, $2$, $3$ such that $2^n$ contains no zero in its decimal representation:
::$2^4 = 16$
:The $5$th integer $n$ after $0$, $1$, $2$, $3$ such that $5^n$ contains no zero in its decimal representation:
::$5^4 = 625$
:The $5$th integer $n$ after $0$, $1$, $2$, $3$ such that both $2^n$ and $5^n$ have no zeroes:
::$2^4 = 16$, $5^4 = 625$
:The $5$th number after $0$, $1$, $2$, $3$ which is (trivially) the sum of the increasing powers of its digits taken in order:
::$4^1 = 4$
=== Miscellaneous ===
:The number of faces and vertices of a tetrahedron
:The number of sides and vertices of a square
:The number of faces which meet at each vertex of a regular octahedron
:The number of dimensions in Einstein's space-time
:There exist exactly $4$ Kepler-Poinsot polyhedra
:Every positive integer can be expressed as the sum of at most $4$ squares.
== Also see ==
* Hyperbola can be Drawn through Four Non-Collinear Points
* Plane Figure with Bilateral Symmetry about Two Lines has 4 Congruent Parts
* Lagrange's Four Square Theorem, also represented as Hilbert-Waring Theorem for $2$nd Powers
* Ferrari's Method
* Four Color Theorem
* Squares which are 4 Less than Cubes
* Four Kepler-Poinsot Polyhedra
* Brocard's Problem
* Divisibility of n-1 Factorial by Composite n‎
* Alternating Group is Simple except on 4 Letters
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=== Next in Sequence: $5$ ===
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=== Next in Sequence: $7$ ===
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=== Next in Sequence: $9$ ===
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=== Next in Sequence: $10$ and above ===
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== Historical Note ==
== Linguistic Note ==
== Sources ==
* : $4$
* : $4$
Category:Specific Numbers
Category:4