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If the number is too large, you can also break it down into several strings with e digits each, satisfying either 10 e = 1 or 10 e = −1 (mod D). The sum (or alternating sum) of the numbers have the same divisibility as the original one. For example, to determine whether 913 = 10 × 91 + 3 is divisible by 11, find that m = (11 × 9
d() is the number of positive divisors of n, including 1 and n itself; σ() is the sum of the positive divisors of n, including 1 and n itselfs() is the sum of the proper divisors of n, including 1 but not n itself; that is, s(n) = σ(n) − n
In number theory, two integers a and b are coprime, relatively prime or mutually prime if the only positive integer that is a divisor of both of them is 1. [1] Consequently, any prime number that divides a does not divide b, and vice versa. This is equivalent to their greatest common divisor (GCD) being 1. [2] One says also a is prime to b or a ...
Zelinsky proved that no three consecutive integers can all be refactorable. [1] Colton proved that no refactorable number is perfect . The equation gcd ( n , x ) = τ ( n ) {\displaystyle \gcd(n,x)=\tau (n)} has solutions only if n {\displaystyle n} is a refactorable number, where gcd {\displaystyle \gcd } is the greatest common divisor function.
Name First elements Short description OEIS Mersenne prime exponents : 2, 3, 5, 7, 13, 17, 19, 31, 61, 89, ... Primes p such that 2 p − 1 is prime.: A000043 ...
A primitive Pythagorean triple is one in which a, b and c are coprime (that is, they have no common divisor larger than 1). [1] For example, (3, 4, 5) is a primitive Pythagorean triple whereas (6, 8, 10) is not. Every Pythagorean triple can be scaled to a unique primitive Pythagorean triple by dividing (a, b, c) by their greatest common divisor ...
Similarly, their respective prime indices (21 and 12) in the list of prime numbers are also permutations of the same digits (1, and 2). 73 is the 21st prime number. It satisfies the "product property" since the product of its decimal digits is precisely in equivalence with its index in the sequence of prime numbers . i.e., 21 = 7 × 3.
For 1<N<10 9, aban numbers are numbers which the integer part of N/1000 is divisible by 1000. Eban numbers. Eban numbers are never odd, due to "one", "three", "five ...