The number two has many properties in mathematics.[1]<span> An </span>integer<span> is called </span>even<span> if it is divisible by 2. For integers written in a numeral system based on an even number, such as </span>decimal<span> and </span>hexadecimal<span>, 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 </span>digital root that is even.3 is:<span><span>a rough approximation of π (3.1415...) and a very rough approximation of e (2.71828..) when doing quick estimates.</span><span>the first odd prime number,[2] and the second smallest prime.</span><span>the first Fermat prime (<span>2<span>2n</span> + 1</span>).</span><span>the first Mersenne prime (<span>2n − 1</span>).</span>the only number that is both a Fermat prime and a Mersenne prime.<span>the first lucky prime.</span><span>the first super-prime.</span><span>the first unique prime due to the properties of its reciprocal.</span><span>the second Sophie Germain prime.</span>the second Mersenne prime exponent.<span>the second factorial prime (2! + 1).</span><span>the second Lucas prime.</span><span>the second Stern prime.[3]</span><span>the second triangular number and it is the only prime triangular number.</span><span>the third Heegner number.[4]</span><span>both the zeroth and third Perrin numbers in the Perrin sequence.[5]</span><span>the fourth Fibonacci number.</span><span>the fourth open meandric number.</span><span>the aliquot sum of 4.</span><span>the smallest number of sides that a simple (non-self-intersecting) polygon can have.</span><span>the only prime which is one less than a perfect square. Any other number which is <span>n2 − 1</span> for some integer n is not prime, since it is <span>(n − 1)(n + 1)</span>. 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 <span>n − 1</span> and <span>n + 1</span> are greater than 1 so their product is not prime.</span><span>the number of non-collinear points needed to determine a plane and a circle.</span></span>