Maths Olympiad Prep

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Number theory Difficulty 5.5 AIME, harder Prove it Ireland

Show that there are 21 consecutive composite four-digit numbers.

Solution

The obvious solution, if we did not care about the bound, is something like N=22!N = 22!. Then, N+kN + k is divisible by kk for k=2,,22k = 2, \dots, 22. However, 22!22! is far too large.

Instead, we take NN to have several small prime factors in order that N+kN + k has small factors for most values of k=2,,22k = 2, \dots, 22, and we then tweak NN to take care of missing factors. We want a small NN so we have to try to be efficient: small factors, and factors that handle several values of kk, are best.

It makes sense to start with taking NN to be a multiple of both 22 and 33, since these factors are small, and together they handle all values of kk except k{5,7,11,13,17,19}k \in \{5, 7, 11, 13, 17, 19\}.

We next assume that N3(mod5)N \equiv 3 \pmod{5} since this is more efficient than assuming that NN is a multiple of 55: the former handles two of the remaining values of kk (namely k=7k=7 and k=17k=17), whereas the latter handles only k=5k=5. This leaves k{5,11,13,19}k \in \{5, 11, 13, 19\} to be handled. For a similar reason, we assume that N2(mod7)N \equiv 2 \pmod{7} in order to handle k=5k=5 and k=19k=19, leaving only k{11,13}k \in \{11, 13\}. Finally, we assume that NN is a multiple of 1111 and of 1313 to handle these two values of kk.

Thus, NN is a multiple of n:=231113=858n := 2 \cdot 3 \cdot 11 \cdot 13 = 858. Note that nn equals 3mod53 \bmod 5, as desired. Thus, we need NN to differ from nn by a multiple of 5n5n. Now, n4(mod7)n \equiv 4 \pmod{7}, and 5n1(mod7)5n \equiv -1 \pmod{7}, so N=11nN = 11n is what we need.

Clearly, NN has at least four digits, so it remains to check that N=2311213N = 2 \cdot 3 \cdot 11^2 \cdot 13 is at most 9999229999 - 22. In fact, it is easier to compare NN with A=9900A = 9900. It is clear that N/66=1113=143N/66 = 11 \cdot 13 = 143, while A/66=150A/66 = 150, so we are done.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.