← back to the spiralPrime factorisation
21
compositeA composite number is greater than 1 and has divisors beyond 1 and itself, so it splits into a product of smaller primes.
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 →21 = 3 × 7
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parity
odd
digits
2
digit sum
The digit sum just adds up the digits — handy for divisibility tricks (a number is divisible by 3 exactly when its digit sum is).
3
divisors τ
τ (tau) counts how many positive divisors a number has, including 1 and itself.
Learn more →4
sum of divisors σ
σ (sigma) is the sum of all positive divisors of a number, including 1 and itself.
Learn more →32
classification
Compare a number to the sum of its proper divisors: equal means perfect, more means abundant, less means deficient.
Learn more →deficient