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In number theory, the Fermat pseudoprimes make up the most important class of pseudoprimes that come from Fermat's little theorem.
A composite number is a positive integer that can be formed by multiplying two smaller positive integers. Accordingly it is a positive integer that has at least one divisor other than 1 and itself. 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 unit. E.g., the integer 14 is a composite number because it is the product of the two smaller integers 2 × 7 but the integers 2 and 3 are not because each can only be divided by one and itself.
21 (twenty-one) is the natural number following 20 and preceding 22.
In number theory, a sphenic number is a positive integer that is the product of three distinct prime numbers. Because there are infinitely many prime numbers, there are also infinitely many sphenic numbers.
In mathematics, a semiprime is a natural number that is the product of exactly two prime numbers. The two primes in the product may equal each other, so the semiprimes include the squares of prime numbers. Because there are infinitely many prime numbers, there are also infinitely many semiprimes. Semiprimes are also called biprimes, since they include two primes, or second numbers, by analogy with how "prime" means "first".
29 (twenty-nine) is the natural number following 28 and preceding 30. It is a prime number.
38 (thirty-eight) is the natural number following 37 and preceding 39.
69 is the natural number following 68 and preceding 70. An odd number and a composite number, 69 is divisible by 1, 3, 23 and 69.
35 (thirty-five) is the natural number following 34 and preceding 36
85 (eighty-five) is the natural number following 84 and preceding 86.
86 (eighty-six) is the natural number following 85 and preceding 87.
87 (eighty-seven) is the natural number following 86 and preceding 88.
55 (fifty-five) is the natural number following 54 and preceding 56.
58 (fifty-eight) is the natural number following 57 and preceding 59.
62 (sixty-two) is the natural number following 61 and preceding 63.
95 (ninety-five) is the natural number following 94 and preceding 96.
213 is the number following 212 and preceding 214.
216 is the natural number following 215 and preceding 217. It is a cube, and is often called Plato's number, although it is not certain that this is the number intended by Plato.
177 is the natural number following 176 and preceding 178.
1234 is the natural number following 1233, and preceding 1235. It has two distinct prime factors, 2 and 617, making it a squarefree semiprime. It is the number of independent vertex sets in a 4×4 square grid, or equivalently, the number of distinct 4×4 binary matrices in which no two adjacent elements are both equal to 1.
References
- ↑ "Sloane's A006881 : Squarefree semiprimes". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.