7

Number
$7$ (seven) is:


 * The $4$th prime number after $2, 3, 5$


 * The $2$nd of the $2$nd pair of twin primes, with $5$


 * The $4$th (trivial, $1$-digit, after $2, 3, 5$) palindromic prime.


 * The $4$th permutable prime after $2, 3, 5$.


 * The smallest prime number of the form $6 n + 1$:
 * $7 = 6 \times 1 + 1$


 * The $2$nd heptagonal number after $1$:
 * $7 = 1 + 7 = \dfrac {2 \left({5 \times 2 - 3}\right)} 2$


 * The $2$nd centered hexagonal number after $1$:
 * $7 = 1 + 6 = 2^3 - 1^3$


 * The $2$nd hexagonal pyramidal number after $1$:
 * $7 = 1 + 6$


 * The start of an arithmetic progression of $6$ prime numbers:
 * $7, 37, 67, 97, 127, 157$


 * The $1$st of the smallest pair of consecutive prime numbers different by $4$


 * The $2$nd Mersenne number and Mersenne prime after $3$, leading to the $2$nd perfect number $28$:
 * $7 = 2^3 - 1$


 * The $4$th Lucas number after $(2), 1, 3, 4$:
 * $7 = 3 + 4$


 * The $2$nd Woodall number after $1$, and $1$st Woodall prime:
 * $7 = 2 \times 2^2 - 1$


 * The $1$st number that is not the sum of at most $3$ square numbers: see Integer as Sum of Three Squares


 * The $3$rd prime number after $2, 3$ to be of the form $n! + 1$ for integer $n$:
 * $3! + 1 = 6 + 1 = 7$
 * where $n!$ denotes $n$ factorial


 * The $3$rd $n$ after $4$ and $5$, and the largest known, such that $n! + 1$ is square: see Brocard's Problem:
 * $7! + 1 = 5040 + 1 = 5041 = 71^2$


 * The $3$rd lucky number:
 * $1, 3, 7, \ldots$


 * The $3$rd palindromic lucky number:
 * $1, 3, 7, \ldots$


 * The $2$nd happy number after $1$:
 * $7 \to 7^2 = 49 \to 4^2 + 9^2 = 16 + 81 = 97 \to 9^2 + 7^2 = 81 + 49 = 130 \to 1^2 + 3^2 + 0^2 = 1 + 9 + 0 = 10 \to 1^2 + 0^2 = 1$


 * The $1$st prime number whose period is of maximum length:
 * $\dfrac 1 7 = 0 \cdotp \dot 14285 \dot 7$


 * The $4$th of the sequence of $n$ such that $p_n \# + 1$, where $p_n \#$ denotes primorial of $n$, is prime, after $2, 3, 5$:
 * $p_7 \# + 1 = 2 \times 3 \times 5 \times 7 + 1 = 211$


 * The $2$nd of $5$ primes of the form $2 x^2 + 5$:
 * $2 \times 1^2 + 5 = 7$


 * The $8$th integer $n$ after $0, 1, 2, 3, 4, 5, 6$ such that both $2^n$ and $5^n$ have no zeroes:
 * $2^7 = 128, 5^7 = 78 \, 125$


 * The $6$th (strictly) positive integer after $1, 2, 3, 4, 6$ which cannot be expressed as the sum of exactly $5$ non-zero squares.


 * The $5$th integer after $0, 1, 3, 5$ which is palindromic in both decimal and binary:
 * $7_{10} = 111_2$


 * The $1$st integer the decimal representation of whose square can be split into two parts which are each themselves square:
 * $7^2 = 49; 4 = 2^2, 9 = 3^2$


 * The $8$th integer $n$ after $0, 1, 2, 3, 4, 5, 6$ such that $2^n$ contains no zero in its decimal representation:
 * $2^7 = 128$


 * The $2$nd positive integer $n$ after $4$ such that $n - 2^k$ is prime for all $k$


 * The $2$nd positive integer after $1$ the sum of whose divisors is a cube:
 * $\sigma \left({7}\right) = 8 = 2^3$


 * The $4$th positive integer which is not the sum of $1$ or more distinct squares:
 * $2, 3, 6, 7, \ldots$


 * The $8$th number after $0, 1, 2, 3, 4, 5, 6$ which is (trivially) the sum of the increasing powers of its digits taken in order:
 * $7^1 = 7$


 * The $7$th of the trivial $1$-digit pluperfect digital invariants after $1, 2, 3, 4, 5, 6$:
 * $7^1 = 7$


 * The $4$th positive integer after $2, 3, 4$ which cannot be expressed as the sum of distinct pentagonal numbers.

Also see

 * Brocard's Problem