Sets Problem!

Let A A be a set with 8 elements. Then the maximal number of 3-element subsets of A A ; such that the intersection of any two of them is not a 2- element set is N . N.

Find N 2 . N^2.

If anyone has a nice solution to this problem, please do post it:)

16 64 36 49 0 25

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1 solution

Otto Bretscher
Dec 6, 2018

With a little experimentation, it isn't hard to find eight such sets, for example, ( 1 , 2 , 3 ) , ( 1 , 4 , 5 ) , ( 1 , 6 , 7 ) , ( 3 , 4 , 8 ) , ( 5 , 6 , 8 ) , ( 2 , 7 , 8 ) , ( 3 , 5 , 7 ) , ( 2 , 4 , 6 ) (1,2,3),(1,4,5),(1,6,7),(3,4,8),(5,6,8),(2,7,8),(3,5,7),(2,4,6) . There cannot be more than eight such sets since no number can appear in more than three. Thus the answer is 64 \boxed{64} .

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