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Dixon's method replaces the condition "is the square of an integer" with the much weaker one "has only small prime factors"; for example, there are 292 squares smaller than 84923; 662 numbers smaller than 84923 whose prime factors are only 2,3,5 or 7; and 4767 whose prime factors are all less than 30. (Such numbers are called B-smooth with ...
The CRT says that this is the same as p ≡ 1 (mod 840), and Dirichlet's theorem says there are an infinite number of primes of this form. 2521 is the smallest, and indeed 1 2 ≡ 1, 1046 2 ≡ 2, 123 2 ≡ 3, 2 2 ≡ 4, 643 2 ≡ 5, 87 2 ≡ 6, 668 2 ≡ 7, 429 2 ≡ 8, 3 2 ≡ 9, and 529 2 ≡ 10 (mod 2521).
In another case, testing p = 13, we have 17 (13 − 1)/2 = 17 6 ≡ 1 (mod 13), therefore 17 is a quadratic residue modulo 13. As confirmation, note that 17 ≡ 4 (mod 13), and 2 2 = 4. We can do these calculations faster by using various modular arithmetic and Legendre symbol properties.
Then () = means that the order of the group is 8 (i.e., there are 8 numbers less than 20 and coprime to it); () = means the order of each element divides 4, that is, the fourth power of any number coprime to 20 is congruent to 1 (mod 20).
Time-keeping on this clock uses arithmetic modulo 12. Adding 4 hours to 9 o'clock gives 1 o'clock, since 13 is congruent to 1 modulo 12. In mathematics, modular arithmetic is a system of arithmetic for integers, where numbers "wrap around" when reaching a certain value, called the modulus.
The number π(B), denoting the number of prime numbers less than B, will control both the length of the vectors and the number of vectors needed. Use sieving to locate π(B) + 1 numbers a i such that b i = (a i 2 mod n) is B-smooth. Factor the b i and generate exponent vectors mod 2 for each one.
The square-free part is 7, the square-free factor such that the quotient is a square is 3 ⋅ 7 = 21, and the largest square-free factor is 2 ⋅ 3 ⋅ 5 ⋅ 7 = 210. No algorithm is known for computing any of these square-free factors which is faster than computing the complete prime factorization.
Primes satisfying (p−1)! ≡ −1 (mod p 2). A007540: Happy numbers: 1, 7, 10, 13, 19, 23, 28, 31, 32, 44, ... The numbers whose trajectory under iteration of sum of squares of digits map includes 1. A007770: Factorial primes: 2, 3, 5, 7, 23, 719, 5039, 39916801, ... A prime number that is one less or one more than a factorial (all factorials ...