101 (number)
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101 is a natural number that follows 100 and precedes 102. It is a prime number—meaning it cannot be divided by any number other than 1 and itself. It is the 26th prime number in sequence, after 97 and before 103.
In base ten, 101 comprises three digits: one hundred, zero tens, and one unit. It is also a palindrome—it reads the same from left to right as from right to left.
Mathematical properties
101 and 103 are twin primes, since they differ by two. It is also one of the numbers called 'unique primes' when considering the decimal expansion of 1/101, whose digits repeat in a cycle of four places (0.00990099...). 101 is an exceptional prime number that is also a palindrome, following 2, 3, 5, 7, and 11.
101 can also be expressed as the sum of two squares: 1² + 10² = 101. Like prime numbers of the form 4n+1, it is possible to represent it this way, and it can be done uniquely.
Numeral systems
In binary, 101 is written as 1100101. In hexadecimal, it is 65. Moreover, when 101 is read in binary (as a binary number), it equals 5 in base ten—which is why the notation '101' is used in computer examples.
Cultural usage
In English, '101' is used to mean 'basics of a subject'. This originates from the course-numbering system at North American universities, where the first course in a department is given the number 101. The term has since become global, as in 'Economics 101'.