13
primeA prime number is a whole number greater than 1 whose only divisors are 1 and itself — the indivisible building block of every other number.
Learn more →Twin primes are a pair of primes that differ by exactly 2, like 11 and 13. Whether there are infinitely many is a famous open question.
Learn more →Prime factorisationEvery whole number above 1 splits into a product of primes in exactly one way — its prime factorisation.
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Every whole number above 1 splits into a product of primes in exactly one way — its prime factorisation.
Learn more →13 = 13
The digit sum just adds up the digits — handy for divisibility tricks (a number is divisible by 3 exactly when its digit sum is).
τ (tau) counts how many positive divisors a number has, including 1 and itself.
Learn more →σ (sigma) is the sum of all positive divisors of a number, including 1 and itself.
Learn more →Compare a number to the sum of its proper divisors: equal means perfect, more means abundant, less means deficient.
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