5
5 (five) is a number, numeral and digit. It is the natural number, and cardinal number, following 4 and preceding 6, and is a prime number. It has garnered attention throughout history in part because distal extremities in humans typically contain five digits.
The evolution of the modern Western digit for the numeral 5 cannot be traced back to the Indian system, as for the digits 1 to 4. The Kushana and Gupta empires in what is now India had among themselves several forms that bear no resemblance to the modern digit. The Nagari and Punjabi took these digits and all came up with forms that were similar to a lowercase "h" rotated 180°. The Ghubar Arabs transformed the digit in several ways, producing from that were more similar to the digits 4 or 3 than to 5.[1] It was from those digits that Europeans finally came up with the modern 5.
While the shape of the character for the digit 5 has an ascender in most modern typefaces, in typefaces with text figures the glyph usually has a descender, as, for example, in 
.
On the seven-segment display of a calculator and digital clock, it is represented by five segments at four successive turns from top to bottom, rotating counterclockwise first, then clockwise, and vice-versa. It is one of three numbers, along with 4 and 6, where the number of segments matches the number.
Five is the third-smallest prime number, and the second super-prime.[2] It is the first safe prime,[3] the first good prime,[4] the first balanced prime,[5] and the first of three known Wilson primes.[6] Five is the second Fermat prime,[2] the second Proth prime,[7] and the third Mersenne prime exponent,[8] as well as the third Catalan number[9] and the third Sophie Germain prime.[2] Notably, 5 is equal to the sum of the only consecutive primes 2 + 3 and it is the only number that is part of more than one pair of twin primes, (3, 5) and (5, 7).[10][11] It also forms the first pair of sexy primes with 11,[12] which is the fifth prime number and Heegner number,[13] as well as the first repunit prime in decimal; a base in-which five is also the first non-trivial 1-automorphic number.[14] Five is the third factorial prime,[15] and an alternating factorial.[16] It is also an Eisenstein prime (like 11) with no imaginary part and real part of the form .[2] In particular, five is the first congruent number, since it is the length of the hypotenuse of the smallest integer-sided right triangle.[17]
5 is the fifth Fibonacci number, being 2 plus 3,[2] and the only Fibonacci number that is equal to its position aside from 1 (that is also the second index). Five is also a Pell number and a Markov number, appearing in solutions to the Markov Diophantine equation: (1, 2, 5), (1, 5, 13), (2, 5, 29), (5, 13, 194), (5, 29, 433), ... (OEIS: A030452 lists Markov numbers that appear in solutions where one of the other two terms is 5). In the Perrin sequence 5 is both the fifth and sixth Perrin numbers.[18]