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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".
114 is the natural number following 113 and preceding 115.
300 is the natural number following 299 and preceding 301.
360 is the natural number following 359 and preceding 361.
400 is the natural number following 399 and preceding 401.
500 is the natural number following 499 and preceding 501.
700 is the natural number following 699 and preceding 701.
600 is the natural number following 599 and preceding 601.
800 is the natural number following 799 and preceding 801.
900 is the natural number following 899 and preceding 901. It is the square of 30 and the sum of Euler's totient function for the first 54 positive integers. In base 10, it is a Harshad number. It is also the first number to be the square of a sphenic number.
In number theory, a nontotient is a positive integer n which is not a totient number: it is not in the range of Euler's totient function φ, that is, the equation φ(x) = n has no solution x. In other words, n is a nontotient if there is no integer x that has exactly n coprimes below it. All odd numbers are nontotients, except 1, since it has the solutions x = 1 and x = 2. The first few even nontotients are this sequence:
In mathematics, an untouchable number is a positive integer that cannot be expressed as the sum of all the proper divisors of any positive integer. That is, these numbers are not in the image of the aliquot sum function. Their study goes back at least to Abu Mansur al-Baghdadi, who observed that both 2 and 5 are untouchable.
182 is the natural number following 181 and preceding 183.
230 is the natural number following 229 and preceding 231.
290 is the natural number following 289 and preceding 291.
318 is the natural number following 317 and preceding 319.
258 is the natural number following 257 and preceding 259.
278 is the natural number following 277 and preceding 279.
286 is the natural number following 285 and preceding 287.
310 is the natural number following 309 and preceding 311.
References
- ↑ "Facts about the integer". Wolfram MathWorld.
- ↑ "Sphenic numbers". Wolfram MathWorld.
- ↑ "Nontotient numbers". Wolfram MathWorld.
- ↑ "Inconsummate numbers". oeis.org.