# 2

This number is a prime.

The first prime number.

The ancient Greek mathematicians started counting at 2. [Lenstra]

Fermat's Little Theorem tells us that if *p* is prime, then *p* divides 2^{p} - 2.

Every number of the form 2^{p-1}(2^{p} - 1), where 2^{p} - 1 is prime, is an even perfect number.

A "goody-*two*-shoes" is someone who thinks they are perfect.

Consider the first 2 primes, i.e., 2 and 3. It is interesting that 2^{10} is quite close to 10^{3} in base 10. [Wells]

The number of odd entries in the *n*th row of Pascal's triangle
is 2 raised to the number of ones in the binary expansion of *n*. [Su]

The number of representations of *n* as the sum of two primes is, at most, the number of primes in the interval [*n*/2, *n* - 2]. [MOC]

Divisibility test for 2: A number is divisible by 2 if it ends in 0, 2, 4, 6, or 8.

2! + 2 = 2^{2}. [Sladcik]

If a polygon has *n* sides, then *n* - 2 triangles are formed. [Glencoe]

Every Fermat number is the product of all previous Fermat numbers plus 2.

The smallest untouchable number, i.e., an integer that cannot be expressed as the sum of all the proper divisors of any positive integer (including the untouchable number itself). The first few are 2, 5, 52, 88, 96, ....

2^2^2^2 - 2^2^2 divides *n*^2^2^2 - *n*^2^2 for all *n*. [Selfridge]

Euler's formula: V - E + F = 2. For any convex polyhedron, the number of vertices and faces together is exactly two more than the number of edges.

The probability that the greatest prime factor of a random integer *n* is greater than the square root of *n* equals the natural logarithm of 2. [Schroeppel]

2 is the only prime digit p with prime number_of_letters (nol) in p, nol = 3, and prime sum p + nol = 5. [Seidov]

Fermat's Last Theorem: The equation *x*^{n} + *y*^{n} = *z*^{n} has no solution in positive integers for *n* greater than 2. [Wiles]

If 2^{p} - 1 is prime then p is prime. [Murthy]

The length of the hypotenuse is 2 times the shorter leg in a 30-60-90 triangle. [Sargent]

The smallest prime of the form 2^{p} - p. [Capelle]

The Pythagoreans considered 2 to be the first feminine number.

The Pythagoreans were embarrassed by the discovery that the square root of 2 is an irrational number, so they tried to keep this fact a secret. [Malinin]

2 is the only prime of form *n*^{n} + *n*. [Luhn]

2π*r* represents the circumference of a circle of radius *r*. [Apostol]

The only difference between two consecutive primes which is prime. [Luhn]

The only even prime number, therefore the "oddest" prime of all.

Any number with only two positive divisors is a prime number. [Gupta]

Bertrand's Postulate states that there is always a prime between *n* and 2*n*.

You need only one hand to slap but 2 to clap. [Murthy]

(2^2^2 + 1)^(2^2^2 + 1) - (2^2^2 + 1) - 1 is a 21 digit prime. [Luhn]

The number of authors of the Even-Mansour Cipher is 2, which is odd, because one of the authors is Even. [Croll]

The only prime which is the average of two consecutive terms of the Lucas sequence. [Rupinski]

The floor function of phi^phi = 2, where phi is Golden Ratio. [Gupta]

UCLA mathematician and prime number researcher Terence Tao taught himself arithmetic at age 2. When his parents asked him how he had learnt these skills, he replied that he had been watching Sesame Street on television.

The first 2 primes are the only 2 primes which are minimal primes in all bases. [Rupinski]

Given any even digit E and any odd digit O, integer D, and 0 ≤ R < 2^{D}, there is exactly one number D digits in length containing only the digits E and O which leaves remainder R when divided by 2^{D}. [Rupinski]

It can be shown that the probability that the greatest prime factor of a random integer n is greater than sqrt(n) is ln 2. [Rupinski]

The shortest possible game of chess ending in checkmate (Fools Mate) has only 2 moves played by each side. [Patterson]

2! is the only factorial that is prime.

There is no prime between n! + 2 and n! + n. [Capelle]

Pseudoprimes to base 2 are sometimes called Poulet numbers.

!2 + 1 is prime. Note that !2 represents subfactorial 2. [Gupta]

The smallest field in abstract algebra has 2 elements.

2^2 - 1 is the first Mersenne prime. [Rajh]

The smallest dihedral prime. [Patterson]

The only prime p such that p times reversal(p) - 1 is prime. [Firoozbakht]

"When you're one step ahead of the crowd you're a genius. When you're 2 steps ahead, you're a crackpot." Rabbi Shlomo Riskin

2 is the smallest Kynea prime. [McAlee]

Is it true that only 2 books of the King James Version of the Bible end in a question mark? Yes, the books of Jonah and Nahum.

The first prime Bell number. Such numbers represent the number of ways a set of *n* elements can be partitioned into nonempty subsets. Named after Eric Temple Bell, a prolific Caltech math professor. [Post]

2 is the smallest prime Motzkin number. [Post]

The addition and product of 2 with itself are equal, which gives it a unique arithmetic property among the positive integers.

Mars is the only known planet to have two natural satellites. [Brower]

De Polignac's Conjecture states that every even number is the difference of 2 consecutive primes in infinitely many ways.

π(*n*) is greater than or equal to π(*n*/2)3/2 for each natural number *n*. Note that 2 and 3 are the first prime numbers. [Capelle]

The number of words in the shortest verse (by number of letters) in the King James Version of the Bible (John 11:35) is "Jesus wept." [Doyle]

The limiting sum in the infinite series of the reciprocal of triangular numbers. [Beedassy]

The only "eban" prime, i.e., devoid of the letter 'e' in its English name. [Beedassy]

A knight in the corner of the chessboard has only 2 possible moves. [Silva]

The only prime that is common divisor to all odd primes gaps. [Silva]

If *p* and *q* are consecutive prime numbers, then floor(*p*/*q* + *q*/*p*) = 2. Note that lim(*p*/*q* + *q*/*p*) = 2, as *p* and *q* approach infinity. [Capelle]

The only prime whose cube is the sum of it s two consecutive primes. [Silva]

The *phase rule* states that the number of degrees of freedom
in a material system at equilibrium is equal to the number
of components minus the number of phases plus the constant
2.

π(2^{π}) = π(π^{2}). [Capelle]

The only known number *n* such that the sum of the proper divisors of *n* is equal to phi(*n*). [Firoozbakht]

Logicians Bertrand Russell (1872-1970) and Alfred Whitehead
(1861-1947) went to great lengths to prove that 1 + 1 = 2,
in their epochal *Principia Mathematica*.

F. Viete (1540-1603) expressed π as an infinite product containing only 2 (and its reciprocal 1/2). [Caldwell]

It is possible to measure all of the integer distances from one to six on a six-inch ruler with just 2 marks. For example, the distance from the 2 to the right end is four inches. [Caldwell]

The only prime digit whose complement is a nonprime digit. [Beedassy]

The only digit d that appears exactly d times in d-digit primes. [Silva]

The smallest number and only prime such that prime(n)=sigma(n). [Gupta]

The smallest prime with a prime number of partitions. [Pol]

"Check this out. The second derivative of *e*^{x} is *e*^{x}, right? And *e*^{x} evaluated at 0 is equal to 1, right? Therefore 2 has got to be a prime number." (from the paper *A Curious Way to Test for Primes* by Dennis P. Walsh, 2007)

*The Solitude of Prime Numbers* by Paolo Giordano is a quiet
book about 2 damaged people, Alice and Mattia.

The only prime number that is not the difference of two squares. [Green]

Only prime whose sum of divisors is odd. [Gupta]

Only prime whose sum of divisors is prime. [Firoozbakht]

5^{3/7} is close to 2. [Wesolowski]

The novel "Curios" by Richard Marsh is about some strange adventures of 2 bachelors.

The smallest prime number which is palindromic in Latin script: II. [Vincent]

Anyone with an IQ in the top 2 percent of the population can join Mensa.

The only prime pronic number. [Homewood]

The first of only four all-Niven numbers that are primes. [Loungrides]

The sum of the reciprocals of the divisors of a perfect number (including the reciprocal of the number itself) is always equal to 2. [Rupinski]

There are about 2 lunar eclipses per year. [NASA]

Pollen grains are tiny (only 2 cells). [Nowicki]

The smallest n where the sum of the nth prime + the nth composite number - 4, the sum of their squares - 4, the sum of their cubes - 4 and the sum of their fourth powers - 4 are all prime [Homewood]

There exists a periodic curve based on prime numbers intersected by only two curves.

The Fibonacci sequence had been described centuries earlier in Indian mathematics by Pingala, who did work on enumerating possible patterns of Sanskrit poetry formed from syllables of 2 lengths. [Chowdhury]

Two wrongs do not make a right. [Gupta]

The only known prime x such that 2^x+3^x, 2^x+5^x, 2^x+7^x are all primes, i.e., 13, 29, 53. [Loungrides]

2 is the sample standard deviation of the first three odd primes. [Honaker]

Good Friday occurs 2 days before Easter Sunday in the United States.

The most feared number by primonumerophobics is 2, as it is the lone even prime number and closest to being lonely in quantity. [Wiki]

The sum of the terms in the open interval between 2 and 8 (i.e., 3+4+5+6+7) is 25, a prime square, and 25+2 = 3^3 = 27, a prime cube. Is 2 the ONLY number for this to be true? [Bergot]

A family room of the Prime City Resort Hotel in Angeles City, Philippines, contains 2 "twin size" beds.

In general, you should borrow no more than 2 times your annual gross income. [Curry]

It has been proven that 2 is the only even prime number. Therefore, it stands to reason that its reciprocal (1/2) also plays an infinite role in the Riemann Hypothesis. [McAlee]

The p in "p-adic" is a variable and may be replaced with a prime (yielding, for instance, "the 2-adic numbers"). [Richeson]

(2^3*3^2) +/- 1 and (2^3*3^2)^2 +/- 1 both form a twin-prime pair. [Galliani]

168 is the largest power of the prime 2 whose decimal expansion is free of 2: 2^168 = 374144419156711147060143317175368453031918731001856. Found by Phil Carmody. [Rivera]

There are 2 books in the Bible that are named after women (Ruth and Esther). [Reynolds]

There are 2 people in the Bible (Enoch and Elijah) that never tasted death but were taken to heaven. [Reynolds]

The only oblong prime. [Gupta]

The 20th happy number is 10^2 and the sum of the first 20 happy numbers is 2^10. [Gaydos]

The smallest difference between two palindromic times (9:59:59 and 10:00:01) is 2 seconds. [Gupta]

The sum of the alternating harmonic series is equal to the natural logarithm of 2. [Basel]

In Quantum Field Theory it takes two rotations of space for a spinor (whose spin is 1/2) to return to its original state. [Harrison]