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Worldwide distribution of country calling codes. Regions are coloured by first digit. Country calling codes, country dial-in codes, international subscriber dialing (ISD) codes, or most commonly, telephone country codes are telephone number prefixes for reaching telephone subscribers in foreign countries or areas via international telecommunication networks.
Mobile phones use geographic area codes (two digits): after that, all numbers assigned to mobile service have nine digits, starting with 6, 7, 8 or 9 (example: 55 15 99999–9999). 90 is not possible, because collect calls start with this number.
This is a list of articles about prime numbers. A prime number (or prime) is a natural number greater than 1 that has no positive divisors other than 1 and itself. By Euclid's theorem, there are an infinite number of prime numbers. Subsets of the prime numbers may be generated with various formulas for primes.
27 has an aliquot sum of 13 [2] (the sixth prime number) in the aliquot sequence (27, 13, 1, 0) of only one composite number, rooted in the 13-aliquot tree. [ 3 ] In the Collatz conjecture (i.e. the 3 n + 1 {\displaystyle 3n+1} problem), a starting value of 27 requires 3 × 37 = 111 steps to reach 1, more than any smaller number.
The progressions of numbers that are 0, 3, or 6 mod 9 contain at most one prime number (the number 3); the remaining progressions of numbers that are 2, 4, 5, 7, and 8 mod 9 have infinitely many prime numbers, with similar numbers of primes in each progression.
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