101 (number)

101 (one hundred [and] one) is the natural number following 100 and preceding 102. It is variously pronounced "one hundred and one" / "a hundred and one", "one hundred one" / "a hundred one", and "one oh one". As an ordinal number, 101st (one hundred [and] first), rather than 101th, is the correct form. 101 is: the 26th prime number and the smallest above 100. a palindromic number in decimal, and so a palindromic prime. a Chen prime since 103 is also prime, with which it makes a twin prime pair. a sexy prime since 107 and 113 are also prime, with which it makes a sexy prime triplet. a unique prime because the period length of its reciprocal is unique among primes. an Eisenstein prime with no imaginary part and real part of the form . the fifth alternating factorial. a centered decagonal number. the only existing prime with alternating 1s and 0s in decimal and the largest known prime of the form . the number of compositions of 12 into distinct parts. Given 101, the Mertens function returns 0. It is the second prime to have this property. For a 3-digit number in decimal, this number has a relatively simple divisibility test. The candidate number is split into groups of four, starting with the rightmost four, and added up to produce a 4-digit number. If this 4-digit number is of the form (where a and b are integers from 0 to 9), such as 3232 or 9797, or of the form , such as 707 and 808, then the number is divisible by 101. On the seven-segment display of a calculator, 101 is both a strobogrammatic prime and a dihedral prime. In mineralogy, a Miller index of 101 is a crystal face that crosses the horizontal axis (a) and 3D vertical axis (c) but does not cross the 2D vertical axis (b). In physics and chemistry, it is the atomic number of mendelevium, an actinide. In astronomy it is the Messier designation given to the Pinwheel Galaxy in Ursa Major. According to Books in Print, more books are now published with a title that begins with '101' than '100'. They usually describe or discuss a list of items, such as 101 Ways to.