# 9

This number is a composite.

If the digit sum of *n*!, S(*n*!), is the product of 9 and any prime larger than *n*, then S(*n*!) cannot divide *n*!.

All digits of the prime 2 * 10^{3020} - 1 are 9 except 1. It contains 3021 digits. [Williams]

The sum of the first 9 consecutive prime numbers is a perfect square. [Honaker]

If odd perfect numbers exist, they are of the forms 12*n* + 1 ... or 36*n* + 9. [Touchard]

9 is the smallest "April Fool prime." The sequence begins 9, 21, 27, 33, 39, 49, 51, 57, 63, 69, 77, 81, 87, 91, 93, 99, 111, ... .

For every prime *p* with *p* not equal to 2 and *p* not equal to 5,
there is some number with all digits equal to 9 such that *p* divides
evenly into this number.

Goldbach conjectured that every odd integer greater than or equal to 9 can be represented as the sum of three odd primes.

There are no consecutive-digit primes starting with 9 with digits in descending order. [Madachy]

Define a certain number of irregularly marked points, *n*, along the rim
of a paper circle, then cut along straight lines that join all possible
pairs of points. If *n* = 9, a prime number of separate pieces will be
created. 163 to be exact!

The smallest odd Giuga number must have at least 9 prime factors.

If *a* is greater than *b*, and *b* is greater than or equal to 1, then
*a*^{n} + *b*^{n} has a primitive prime factor with the
exception of 2^{3} + 1^{3} = 9.

9 times the 9th prime has a sum of digits equal to 9.

There are no clusters (groups) of 9 twin prime pairs less than 10^{14}. [DeVries]

Washington University in St. Louis provides a page that calculates the prime factors of a number (with a maximum of 9 digits).

There are at least 9 prime numbers between x^{3} and (x + 1)^{3} for x greater than or equal to π, assuming the Riemann Hypothesis is true.

The smallest odd composite number. [Gupta]

10^{9} + 9 is prime. [Gupta]

Two raised to the 9th power plus and minus 9 are primes! [Hoefakker]

The first digit to appear as an end-digit in two consecutive primes (139 and 149). [Silva]

19, 109, 1009 and 10009 are primes. No other digit can replace the 9 and yield four primes. [Friend]

The number of known positive integers which are the sum of two primes in exactly two ways is a prime square. [Capelle]

2^^n-9 = 2^(2^(2^(....(2^2)...)))-9 is (for large enough n) always divisible by both 7 and 11. Note that 9 is midway between 7 and 11. [Hartley]

There are exactly 3=(sqrt(9)) pandigital improper fractions that reduce to 9 (provided each digit is used once). [Patterson]

9 is the only number m such that m = π(π(m)!). [Firoozbakht]

The 9th Fibonacci number plus 9 is prime. [Losnak]

The only composite digit that can appear as end-digit of a prime. [Silva]

The only non-prime digit that is the difference of consecutive squares. [Silva]

10*(2^{2n} + 1) + 9 gives
primes for n = 1 to 7. Therefore, there are 7 known Fermat numbers which yields
primes when a 9 is appended. [Wesolowski]

The smallest composite number n such that both
2^{n}+n and 2^{n}-n are prime. That is,
2^{9}+9 = 521 and 2^{9}-9 = 503 are prime. [Poo Sung]

Three more semiprimes can be consecutively formed from 9 by iterating the process described in A227942.

The smallest semiprime such that all permutations of concatenations with its factors are semiprimes. [Sariyar]

The only known integer n, such that 2^n-n^2 and 2^n+n^2 are both primes, i.e., 431 and 593. [Loungrides]

Regular nonagon is the only regular polygon such that n^n +(n-2)*180/n (387420629) and ((n-2)*180/n)^n+n (20661046784000000009) are both prime. Note that (n-2)*180/n is an internal angel of a regular polygon. [Sariyar]

Smallest composite number n whose number of circular loops equals omega(n), i.e., the number of distinct primes dividing n. Circular loops occur in the digits 0, 6, 8, or 9 only. Note that the digit 8 contains two loops. The sequence begins 9, 16, 18, 28, ... . [Honaker]

The composite digit which first appears in a prime. [Silva]

The smallest composite number whose both concatenations with its home prime, (i.e., 311), in order and reverse order, 9311 and 3119 are primes. [Loungrides]