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9 is the fourth composite number, and the first odd composite number. 9 is also a refactorable number. [2] Casting out nines is a quick way of testing the calculations of sums, differences, products, and quotients of integers in decimal, a method known as long ago as the 12th century. [3]
Even and odd numbers: An integer is even if it is a multiple of 2, and is odd otherwise. Prime number: A positive integer with exactly two positive divisors: itself and 1. The primes form an infinite sequence 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, ...
Even and odd numbers have opposite parities, e.g., 22 (even number) and 13 (odd number) have opposite parities. In particular, the parity of zero is even. [2] Any two consecutive integers have opposite parity. A number (i.e., integer) expressed in the decimal numeral system is even or odd according to whether its last digit is even or odd. That ...
The number 9. Fear of the number 9 is known as enneaphobia, in Japanese culture; this is because it sounds like the Japanese word for "suffering". [4] [5] The number 13. Fear of the number 13 is known as triskaidekaphobia. The number 17. Fear of the number 17 is known as heptadecaphobia and is prominent in Italian culture. [6] The number 39.
Thus, non-trapezoidal polite number must have the form of a power of two multiplied by an odd prime. As Jones and Lord observe, [12] there are exactly two types of triangular numbers with this form: the even perfect numbers 2 n − 1 (2 n − 1) formed by the product of a Mersenne prime 2 n − 1 with half the nearest power of two, and
As 100=10 2, these are two decimal digits. 121: Number expressible with two undecimal digits. 125: Number expressible with three quinary digits. 128: Using as 128=2 7. [clarification needed] 144: Number expressible with two duodecimal digits. 169: Number expressible with two tridecimal digits. 185
A cluster prime is a prime p such that every even natural number k ≤ p − 3 is the difference of two primes not exceeding p. 3, 5, 7, 11, 13, 17, 19, 23, ... (OEIS: A038134) All odd primes between 3 and 89, inclusive, are cluster primes. The first 10 primes that are not cluster primes are: 2, 97, 127, 149, 191, 211, 223, 227, 229, 251.
If the hundreds digit is even, the number formed by the last two digits must be divisible by 8. 624: 24. If the hundreds digit is odd, the number obtained by the last two digits must be 4 times an odd number. 352: 52 = 4 × 13. Add the last digit to twice the rest. The result must be divisible by 8. 56: (5 × 2) + 6 = 16.