Question

In: Advanced Math

11. (6 Pts.) Show that if we split any 11 numbers in 5 sets, then there...

11. (6 Pts.) Show that if we split any 11 numbers in 5 sets, then there exists one set that contains a subset such
that the sum of its elements is a multiple of 3.

Solutions

Expert Solution

Let be any integers.

Case I: If some divides one of , say , then the set that contains has as its subset, and the sum of all elements in is just , a multiple of .

Case II: Suppose for all : Since sets contains integers, by Pigeon-hole principle, at least one of these sets contains at least of these integers . Say is such a set and . Since for all , we have . Thus, the sum of the elements in the subset is a multiple of .

Case III: Suppose for all : Since sets contains integers, by Pigeon-hole principle, at least one of these sets contains at least of these integers . Say is such a set and . Since for all , we have . Thus, the sum of the elements in the subset is a multiple of .

Case IV: Suppose for at least one of , and for at least one of :

Case IVa If one of the sets contain an and , then it has a two-elements subset whose elements sum is .

Case IVb Since sets contains integers, by Pigeon-hole principle, at least one of these sets contains at least of these integers . Say is such a set and . Either for all , or for all . Thus, we have . Thus, the sum of the elements in the subset is a multiple of .

Since these are all the possible cases, we have proved the required statement.


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