Maths Olympiad Prep

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Number theory Difficulty 5.3 AIME, harder Find the answer

## Task 10/85

We are looking for all quadruples (n1;n2;n3;n4)\left(n_{1} ; n_{2} ; n_{3} ; n_{4}\right) of natural numbers ni(i=1;2;3;4)n_{i}(i=1 ; 2 ; 3 ; 4) with n1<n2<n3<n_{1}<n_{2}<n_{3}< n4n_{4} (where 0N0 \in N), for which

z=n1!n2!n3!n4!32 z=n_{1}!\cdot n_{2}!\cdot n_{3}!\cdot n_{4}!-32

is a perfect square.

A number or a short expression. Spacing and $ signs are ignored.

Solution

If n45n_{4} \geq 5, then n4!n_{4}! contains the prime factors 2 and 5. Therefore,

z=n1!n2!n3!n4!320328(mod10) z=n_{1}!n_{2}!n_{3}!n_{4}!-32 \equiv 0-32 \equiv 8(\bmod 10)

However, (10a+b)2b2(mod10)(10 a+b)^{2} \equiv b^{2}(\bmod 10) holds for a;bNa ; b \in N. If b=0;1;2;3;4;5;6;7;8;9b=0 ; 1 ; 2 ; 3 ; 4 ; 5 ; 6 ; 7 ; 8 ; 9, then b2b^{2} \equiv 0;1;4;9;6;5;6;9;4;1(mod10)0 ; 1 ; 4 ; 9 ; 6 ; 5 ; 6 ; 9 ; 4 ; 1(\bmod 10).

In no case does b28(mod10)b^{2} \equiv 8(\bmod 10) hold. Therefore, n44n_{4} \leq 4. Thus, initially, only the following five quadruples are possible:

(0;1;2;3),(0;1;2;4),(0;1;3;4),(0;2;3;4),(1;2;3;4) (0 ; 1 ; 2 ; 3),(0 ; 1 ; 2 ; 4),(0 ; 1 ; 3 ; 4),(0 ; 2 ; 3 ; 4),(1 ; 2 ; 3 ; 4)

Testing reveals that the first and third quadruples are invalid. Therefore, the solution set contains three elements: (0;1;2;4),(0;2;3;4)(0 ; 1 ; 2 ; 4),(0 ; 2 ; 3 ; 4), and (1;2;3;4)(1 ; 2 ; 3 ; 4).

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.