Maths Olympiad Prep

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Number theory Difficulty 5.4 AIME, harder Prove it Ukraine

We enumerate all prime numbers in ascending order: p1=2,p2=3,p3=5,p_1 = 2, p_2 = 3, p_3 = 5, \dots. Find all positive integer nn, for which p1!+p2!++pn!=abp_1!+p_2!+\dots+p_n!=a^b, for some positive integer a,b>1a, b>1, where k!k! denotes the product of all integers from 1 to kk.

Solution

Directly checking yields:
p1!=2,p1!+p2!=8=23,p1!+p2!+p3!=128=27 p_1! = 2, \quad p_1! + p_2! = 8 = 2^3, \quad p_1! + p_2! + p_3! = 128 = 2^7
For n4n \ge 4, p1!+p2!++pn!=128+7!+11!++pn!p_1! + p_2! + \dots + p_n! = 128 + 7! + 11! + \dots + p_n! is a number in which all summands are divisible by 252^5, except p4!=7!=24315p_4! = 7! = 2^4 \cdot 315, which is divisible by 242^4 but not by 252^5. Therefore, this number cannot be a power of 2. If it is a power of another number, then since 242^4 divides it, it can only be 242^4 or x2x^2. Let us show that it cannot be a square. Indeed, 2!+3!++pn!2! + 3! + \dots + p_n! gives a remainder 2 modulo 3, which is impossible for squares.

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