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Twenty-seven is the cube of 3, or the 2nd tetration of 3: 2 3 = 3 3 = 3 × 3 × 3. It is divisible by the number of prime numbers below it . The first non-trivial decagonal number is 27. [1] 27 has an aliquot sum of 13 [2] (the sixth prime number) in the aliquot sequence (27, 13, 1, 0) of only one composite number, rooted in the 13-aliquot tree ...
The same suffix may be used with more than one category of number, as for example the orginary numbers secondary and tertiary and the distributive numbers binary and ternary. For the hundreds, there are competing forms: Those in -gent- , from the original Latin, and those in -cent- , derived from centi- , etc. plus the prefixes for 1 through 9 .
A prime number (or prime) is a natural number greater than 1 that has no positive divisors other than 1 and itself. By Euclid's theorem, there are an infinite number of prime numbers. Subsets of the prime numbers may be generated with various formulas for primes.
Since the greatest prime factor of + = is 157, which is more than 28 twice, 28 is a Størmer number. [3] Twenty-eight is a harmonic divisor number, [4] a happy number, [5] the 7th triangular number, [6] a hexagonal number, [7] a Leyland number of the second kind [8] (), and a centered nonagonal number.
When reading numbers in a sequence, such as a telephone or serial number, British people will usually use the terms double followed by the repeated number. Hence 007 is double oh seven. Exceptions are the emergency telephone number 999, which is always nine nine nine and the apocalyptic "Number of the Beast", which is always six six six.
The number of perfect numbers less than n is less than , where c > 0 is a constant. [53] In fact it is (), using little-o notation. [54] Every even perfect number ends in 6 or 28, base ten; and, with the only exception of 6, ends in 1 in base 9.
An extravagant number has fewer digits than its prime factorization. The first in decimal: 4, 6, 8, 9, 12, 18, 20, 22, 24, 26, 28, 30 (sequence A046760 in the OEIS). An economical number has been defined as a frugal number, but also as a number that is either frugal or equidigital.
So, 6 is a perfect number because the proper divisors of 6 are 1, 2, and 3, and 1 + 2 + 3 = 6. [ 2 ] [ 4 ] There is a one-to-one correspondence between the Mersenne primes and the even perfect numbers, but it is unknown whether there exist odd perfect numbers.