Examples of using Practical number in English and their translations into Russian
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Practical Number at PlanetMath. org.
Every power of two is a practical number.
In the set of all practical numbers there is a primitive set of practical numbers.
Every even perfect number is also a practical number.
Equivalently, the set of all practical numbers is closed under multiplication.
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Therefore, this number must satisfy the characterization of practical numbers.
Let p(x) count how many practical numbers are at most x.
Every even perfect number andevery power of two is also a practical number.
The product of two practical numbers is also a practical number.
By induction, it follows that every primorial satisfies the characterization of practical numbers.
One reason for interest in practical numbers is that many of their properties are similar to properties of the prime numbers. .
Because a primorial is, by definition,squarefree it is also a primitive practical number.
Fibonacci does not formally define practical numbers, but he gives a table of Egyptian fraction expansions for fractions with practical denominators.
More strongly, Srinivasan(1948) observes that other than 1 and 2, every practical number is divisible by 4 or 6 or both.
The maximum practical number of nodes that can be connected to a 10BASE2 segment is limited to 30 with a minimum distance of 50 cm between devices.
More strongly the least common multiple of any two practical numbers is also a practical number.
Practical numbers were used by Fibonacci in his Liber Abaci(1202) in connection with the problem of representing rational numbers as Egyptian fractions.
Fibonacci provides tables of these representations for fractions having as denominators the practical numbers 6, 8, 12, 20, 24, 60, and 100.
The only odd practical number is 1, because if n> 2 is an odd number, then 2 cannot be expressed as the sum of distinct divisors of n.
According to a September 2015 conjecture by Zhi-Wei Sun,every positive rational number has an Egyptian fraction representation in which every denominator is a practical number.
Hausman& Shapiro(1984) showed that there always exists a practical number in the interval for any positive real x, a result analogous to Legendre's conjecture for primes.
Margenstern(1991) conjectured that p(x) is asymptotic to cx/log x for some constant c, a formula which resemblesthe prime number theorem, strengthening the earlier claim of Erdős& Loxton(1979) that the practical numbers have density zero in the integers.
Powers of two trivially satisfy the characterization of practical numbers in terms of their prime factorizations: the only prime in their factorizations, p1, equals two as required.
In number theory, a practical number or panarithmic number is a positive integer n such that all smaller positive integers can be represented as sums of distinct divisors of n.
From the above characterization by Stewart andSierpiński it can be seen that if n is a practical number and d is one of its divisors then n*d must also be a practical number.
A primitive practical number is either practical and squarefree or practical and when divided by any of its prime factors whose factorization exponent is greater than 1 is no longer practical. .
Indeed, theorems analogous to Goldbach's conjecture andthe twin prime conjecture are known for practical numbers: every positive even integer is the sum of two practical numbers, and there exist infinitely many triples of practical numbers x- 2, x, x+ 2.
Several other notable sets of integers consist only of practical numbers: From the above properties with n a practical number and d one of its divisors(that is, d| n)then n*d must also be a practical number therefore six times every power of 3 must be a practical number as well as six times every power of 2.
The original characterisation by Srinivasan(1948) stated that a practical number cannot be a deficient number, that is one of which the sum of all divisors(including 1 and itself) is less than twice the number unless the deficiency is one.
There are also a number of practical obstacles.