Here is an example. Since 45 = 1 + 2 + 3 + … + 9, it is possible to divide 45 people into groups of 1, 2, …, 9 people in each. Let the people of a group be not familiar with the members of their group, but all the people from different groups are friends. Then every person from the group k has 45 − k friends. In this case the task is satisfied, and the total amount of pairs of friends is:
((45*44)/2) - (((2*1)/2)+((3*2)/2)+...+((9*8)/2)) = 870
Now we have to prove that there cannot be any bigger amount of pairs of friends. Fix some k, 0 ≤ k ≤ 44. Let there be a person with exactly k friends. According to the task, any of his friends cannot have exactly k friends. Thus, the number of people with exactly k friends is not bigger than 45 − k.
Denote A0, A1, … ,A44 as the amounts of people with respectively 0, 1, …, 44 friends. As shown before, Ak ≤ 45 − k, and besides A0 + A1 + ⋯ + A44 = 45.
We can estimate the total number of pairs:
S = (1/2)*(0*A0+1*A1+...+44*A44) = (1/2)(A44+(A44+A43)+...+(A44+A43+...+A36)+...+(A44+A43+...+A0)) ≤ (1/2)*(1+(1+2)+...+(1+2+...+9)+45+45+...+45) = (1/2)*(45*44−((9+8+...+2)+(9+8+...+3)+...+9)) = 870
Answer. 870
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