Mancosu - Philosophy of Mathematical Practice (Oxford, 2008).pdf

392 colin mclartyThen the idea is to think of each integer m ∈ Z as a function defined on thisline. For very good reasons, the values of the function over the point (0) arerational numbers while the values over any point (p) are integers modulo p.The integer m ∈ Z is a function whose value at the point (0) is m and value ateach point (p) is m modulo p. For example the integer 9 has9 ≡ 1 (mod 2) 9 ≡ 0 (mod 3)9 ≡ 4 (mod 5) 9 ≡ 2 (mod 7)and we can graph 9 as a function this way:10432 21 10 06…210109…10…9…(2) (3) (5) (7) (11) … (0)The spatial structure of Spec(Z) is determined by the ‘coordinate functions’on it—namely by the integers! But the picture is only suggestive. Rigorousproofs show schemes have structural relations parallel to those in geometry andthese relations return major new arithmetic theorems. We sketch one simpleexample.14.4.2 The Chinese remainder theoremConsider congruences modulo 4 and 6:a ≡ 2 (mod 4) and a ≡ 1 (mod 6)These two have no solution since the first implies a is even and the secondimplies it is odd. On the other hand considera ≡ 1 (mod 4) and a ≡ 3 (mod 6)These agree modulo 2, as both say a is odd, and clearly a = 9isasolution.Then too a = 21 is a solution. Adding any multiple of 12 will give anothersolution since 12 is the smallest common multiple of 4 and 6. One classicalstatement of the Chinese remainder theorem is:Theorem. Take any integer moduli m 1 , m 2 . Then for any integer remainders r 1 , r 2consider the congruences on an unknown integer aa ≡ r 1 (mod m 1 ) and a ≡ r 2 (mod m 2 )