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Combinatorics Difficulty 7.1 National olympiad, round 2 Prove it China

Given a 3×93 \times 9 array AA with each cell containing a positive integer, we say a m×nm \times n (1m3, 1n9)(1 \le m \le 3,\ 1 \le n \le 9) subarray of AA is a “good rectangle” if the sum of the numbers in its cells is a multiple of 1010, and call a 1×11 \times 1 cell of AA “bad” if it is not contained in any “good rectangle”. Find the maximum number of “bad cells” in AA.

Solution

We first claim that the number of “bad cells” in AA is no more than 2525. Otherwise, there will be at most one cell in AA that is not “bad”. Without loss of generality, we assume the cells in the first row of AA are all “bad”. Then let the numbers from top to bottom in the iith column be ai,bi,cia_i, b_i, c_i (i=1,2,,9i = 1, 2, \dots, 9) in turn, and define
Sk=i=1kai,Tk=i=1k(bi+ci),k=1,2,,9, S_k = \sum_{i=1}^{k} a_i, \quad T_k = \sum_{i=1}^{k} (b_i + c_i), \quad k = 1, 2, \dots, 9,
with S0=T0=0S_0 = T_0 = 0. We are going to prove that three number groups S0,S1,,S9S_0, S_1, \dots, S_9, T0,T1,,T9T_0, T_1, \dots, T_9, and S0+T0,S1+T1,,S9+T9S_0 + T_0, S_1 + T_1, \dots, S_9 + T_9 each form a complete set of residues modulo 1010:

If there exist m,n,0m<n9m, n, 0 \le m < n \le 9 such that SmSn(mod10)S_m \equiv S_n \pmod{10}, then
i=m+1nai=SnSm0(mod10), \sum_{i=m+1}^{n} a_i = S_n - S_m \equiv 0 \pmod{10},
which means that the cells in the first row and from columns m+1m+1 to nn form a “good rectangle”. But it is a contradiction to the assumption that the cells in the first row are all “bad”.

If there exist m,n,0m<n9m, n, 0 \le m < n \le 9 such that TmTn(mod10)T_m \equiv T_n \pmod{10}, then
i=m+1n(bi+ci)=TnTm0(mod10). \sum_{i=m+1}^{n} (b_i + c_i) = T_n - T_m \equiv 0 \pmod{10}.
So the cells ranging from rows 22 to 33 and columns m+1m+1 to nn form a “good rectangle”, which means there are at least two cells that are not “bad”. But it is also a contradiction.

In a similar way, we can also prove that there are no m,n,0m<n9m, n, 0 \le m < n \le 9 such that
Sm+TmSn+Tn(mod10). S_m + T_m \equiv S_n + T_n \pmod{10}.
Therefore, we have
k=09Skk=09Tkk=09(Sk+Tk)0+1+2++95(mod10). \begin{aligned} \sum_{k=0}^{9} S_k &\equiv \sum_{k=0}^{9} T_k \equiv \sum_{k=0}^{9} (S_k + T_k) \equiv 0 + 1 + 2 + \cdots + 9 \\ &\equiv 5 \pmod{10}. \end{aligned}
Then
k=09(Sk+Tk)k=09Sk+k=09Tk5+50(mod10). \sum_{k=0}^{9} (S_k + T_k) \equiv \sum_{k=0}^{9} S_k + \sum_{k=0}^{9} T_k \equiv 5 + 5 \equiv 0 \pmod{10}.
It is again a contradiction! Therefore, the number of “bad cells” in AA is no more than 2525.

On the other hand, we can construct a 3×93 \times 9 array in the following and check that each cell in it that does not contain number 1010 is “bad”.

1112111110
111111111
1111011112

Therefore, we find out that the maximum number of “bad cells” in AA is 2525.

25\boxed{25}

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.