In number theory, a natural number is called semiprime if it can be written as the product of two prime numbers. The two numbers do not need to be different. A semiprime can also be the square of a prime number. Such numbers are very useful for cryptography.
Divisibility-based sets of integers |
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| Overview | |
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| Factorization forms | |
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| Constrained divisor sums |
- Perfect
- Almost perfect
- Quasiperfect
- Multiply perfect
- Hemiperfect
- Hyperperfect
- Superperfect
- Unitary perfect
- Semiperfect
- Practical
- Erdős–Nicolas
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| With many divisors | |
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| Aliquot sequence-related |
- Untouchable
- Amicable (Triple)
- Sociable
- Betrothed
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| Base-dependent |
- Equidigital
- Extravagant
- Frugal
- Harshad
- Polydivisible
- Smith
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| Other sets |
- Arithmetic
- Deficient
- Friendly
- Solitary
- Sublime
- Harmonic divisor
- Descartes
- Refactorable
- Superperfect
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