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Take each digit of the number (371) in reverse order (173), multiplying them successively by the digits 1, 3, 2, 6, 4, 5, repeating with this sequence of multipliers as long as necessary (1, 3, 2, 6, 4, 5, 1, 3, 2, 6, 4, 5, ...), and adding the products (1×1 + 7×3 + 3×2 = 1 + 21 + 6 = 28). The original number is divisible by 7 if and only if ...
Demonstration, with Cuisenaire rods, that 1, 2, 8, 9, and 12 are refactorable. A refactorable number or tau number is an integer n that is divisible by the count of its divisors, or to put it algebraically, n is such that (). The first few refactorable numbers are listed in (sequence A033950 in the OEIS) as
Example 1 "John is a king" implies that John is male. So knowing that John is a king is sufficient to knowing that he is a male. Example 2 A number's being divisible by 4 is sufficient (but not necessary) for it to be even, but being divisible by 2 is both sufficient and necessary for it to be even. Example 3
t(n) = C(n + 1, 2) = n(n + 1) / 2 = 1 + 2 + ... + n for n ≥ 1, with t(0) = 0 (empty sum). A000217: Square numbers n 2: 0, 1, 4, 9, 16, 25, 36, 49, 64, 81, ... n 2 = n × n: A000290: Tetrahedral numbers T(n) 0, 1, 4, 10, 20, 35, 56, 84, 120, 165, ... T(n) is the sum of the first n triangular numbers, with T(0) = 0 (empty sum). A000292 ...
Singly even numbers are those with ν 2 (n) = 1, i.e., integers of the form 4m + 2. Doubly even numbers are those with ν 2 (n) > 1, i.e., integers of the form 4m. In this terminology, a doubly even number may or may not be divisible by 8, so there is no particular terminology for "triply even" numbers in pure math, although it is used in ...
Furthermore, if b 1, b 2 are both coprime with a, then so is their product b 1 b 2 (i.e., modulo a it is a product of invertible elements, and therefore invertible); [6] this also follows from the first point by Euclid's lemma, which states that if a prime number p divides a product bc, then p divides at least one of the factors b, c.
In arithmetic and number theory, the least common multiple, lowest common multiple, or smallest common multiple of two integers a and b, usually denoted by lcm(a, b), is the smallest positive integer that is divisible by both a and b. [1] [2] Since division of integers by zero is undefined, this definition has meaning only if a and b are both ...
Two properties of 1001 are the basis of a divisibility test for 7, 11 and 13. The method is along the same lines as the divisibility rule for 11 using the property 10 ≡ -1 (mod 11). The two properties of 1001 are 1001 = 7 × 11 × 13 in prime factors 10 3 ≡ -1 (mod 1001) The method simultaneously tests for divisibility by any of the factors ...