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  2. Composite number - Wikipedia

    en.wikipedia.org/wiki/Composite_number

    A composite number is a positive integer that can be formed by multiplying two smaller positive integers. Equivalently, it is a positive integer that has at least one divisor other than 1 and itself. [ 1][ 2] Every positive integer is composite, prime, or the unit 1, so the composite numbers are exactly the numbers that are not prime and not a ...

  3. Goldbach's conjecture - Wikipedia

    en.wikipedia.org/wiki/Goldbach's_conjecture

    For instance, if m is odd, then n − m is also odd, and if m is even, then n − m is even, a non-trivial relation because, besides the number 2, only odd numbers can be prime. Similarly, if n is divisible by 3, and m was already a prime other than 3, then n − m would also be coprime to 3 and thus be slightly more likely to be prime than a ...

  4. Carmichael number - Wikipedia

    en.wikipedia.org/wiki/Carmichael_number

    Carmichael number. In number theory, a Carmichael number is a composite number ⁠ ⁠ which in modular arithmetic satisfies the congruence relation : for all integers ⁠ ⁠. [1] The relation may also be expressed [2] in the form: for all integers that are relatively prime to ⁠ ⁠. They are infinite in number.

  5. Prime number - Wikipedia

    en.wikipedia.org/wiki/Prime_number

    A prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number. For example, 5 is prime because the only ways of writing it as a product, 1 × 5 or 5 × 1, involve 5 itself. However, 4 is composite because it is a ...

  6. Highly composite number - Wikipedia

    en.wikipedia.org/wiki/Highly_composite_number

    A highly composite number is a positive integer that has more divisors than any smaller positive integer. A related concept is that of a largely composite number, a positive integer that has at least as many divisors as any smaller positive integer. The name can be somewhat misleading, as the first two highly composite numbers (1 and 2) are not ...

  7. Untouchable number - Wikipedia

    en.wikipedia.org/wiki/Untouchable_number

    The number 5 is untouchable, as it is not the sum of the proper divisors of any positive integer: 5 = 1 + 4 is the only way to write 5 as the sum of distinct positive integers including 1, but if 4 divides a number, 2 does also, so 1 + 4 cannot be the sum of all of any number's proper divisors (since the list of factors would have to contain ...

  8. Sieve of Eratosthenes - Wikipedia

    en.wikipedia.org/wiki/Sieve_of_Eratosthenes

    In mathematics, the sieve of Eratosthenes is an ancient algorithm for finding all prime numbers up to any given limit. It does so by iteratively marking as composite (i.e., not prime) the multiples of each prime, starting with the first prime number, 2. The multiples of a given prime are generated as a sequence of numbers starting from that ...

  9. Superabundant number - Wikipedia

    en.wikipedia.org/wiki/Superabundant_number

    Superabundant number. In mathematics, a superabundant number is a certain kind of natural number. A natural number n is called superabundant precisely when, for all m < n : where σ denotes the sum-of-divisors function (i.e., the sum of all positive divisors of n, including n itself). The first few superabundant numbers are 1, 2, 4, 6, 12, 24 ...