The number two has many properties in mathematics.[1] An integer is called even if it is divisible by 2. For integers written in a numeral system based on an even number, such as decimal and hexadecimal, divisibility by 2 is easily tested by merely looking at the last digit. If it is even, then the whole number is even. In particular, when written in the decimal system, all multiples of 2 will end in 0, 2, 4, 6, or 8. In numeral systems based on an odd number, divisibility by 2 can be tested by having a digital root that is even.3 is:a rough approximation of π (3.1415...) and a very rough approximation of e (2.71828..) when doing quick estimates.the first odd prime number,[2] and the second smallest prime.the first Fermat prime (22n + 1).the first Mersenne prime (2n − 1).the only number that is both a Fermat prime and a Mersenne prime.the first lucky prime.the first super-prime.the first unique prime due to the properties of its reciprocal.the second Sophie Germain prime.the second Mersenne prime exponent.the second factorial prime (2! + 1).the second Lucas prime.the second Stern prime.[3]the second triangular number and it is the only prime triangular number.the third Heegner number.[4]both the zeroth and third Perrin numbers in the Perrin sequence.[5]the fourth Fibonacci number.the fourth open meandric number.the aliquot sum of 4.the smallest number of sides that a simple (non-self-intersecting) polygon can have.the only prime which is one less than a perfect square. Any other number which is n2 − 1 for some integer n is not prime, since it is (n − 1)(n + 1). This is true for 3 as well (with n = 2), but in this case the smaller factor is 1. If n is greater than 2, both n − 1 and n + 1 are greater than 1 so their product is not prime.the number of non-collinear points needed to determine a plane and a circle.
