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Combinatorics Difficulty 7.2 National olympiad, round 2 Prove it Romania

Let NN be a positive integer. On a board, we initially have two numbers: a red 00 and a blue 00. We define the following procedure:

At each step, a natural number kk is chosen (not necessarily distinct from previous choices). Let xx be the blue number and yy the red number. Replace them with:
xx+k+1,yy+k2+2, x \rightarrow x + k + 1, \quad y \rightarrow y + k^2 + 2,
and color the new values blue and red, respectively. This process continues until the blue number is at least NN.
Determine the minimum possible value of the red number at the end of the process.

Solution

Let MkM_k denote the move that increases the blue number by k+1k+1 and the red number by k2+2k^2+2. We show that for any k2k \ge 2, the move MkM_k can be replaced by moves involving only M0M_0 and M1M_1 in such a way that the total increase in the blue number is the same, but the red number increases by less than k2+2k^2+2.

Case 1: k=2p+1k = 2p + 1 is odd. Then MkM_k increases the blue number by 2p+22p + 2 and the red number by (2p+1)2+2(2p+1)^2 + 2. Instead, p+1p+1 moves of type M1M_1 increase the blue number also by 2p+22p+2, but the red number increases by 3(p+1)3(p+1). Since (2p+1)2+2>3(p+1)(2p+1)^2 + 2 > 3(p+1), for all p1p \ge 1, we can replace MkM_k with p+1p+1 moves of M1M_1.

Case 2: k=2pk = 2p is even. Then MkM_k increases the blue number by 2p+12p+1, and the red number by (2p)2+2(2p)^2 + 2. Alternatively, pp moves of M1M_1 and one move of M0M_0 increase the blue number by 2p+12p+1 and the red number by 3p+23p+2. Since (2p)2+2>3p+2(2p)^2 + 2 > 3p+2, for all p1p \ge 1, we conclude that MkM_k can be replaced by pp moves of M1M_1 and one M0M_0.
Moreover, observe that two moves of M0M_0 can be replaced by one move of M1M_1, with a smaller increase in the red number: 33 instead of 44.

Thus, using only M0M_0 and M1M_1, and minimizing the use of M0M_0, yields the minimal red number. To make the blue number at least NN, the best strategy is to use only moves of type M1M_1 (which increase blue by 22 and red by 33), plus at most one M0M_0 (if needed).

If NN is even: we can use N2\left\lfloor \frac{N}{2} \right\rfloor moves of M1M_1, increasing red by 3N2=3N23 \cdot \frac{N}{2} = \frac{3N}{2}.

If NN is odd: we can use N2\left\lceil \frac{N}{2} \right\rceil moves of M1M_1 and one move of M0M_0, leading to red increasing by 3N2+23\left\lfloor \frac{N}{2} \right\rfloor + 2.

Hence, the minimal possible value of the red number is N+N+12N + \left\lfloor \frac{N+1}{2} \right\rfloor.

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