Number theoryDifficulty 5.4AIME, harderProve itUkraine
We enumerate all prime numbers in ascending order: p1=2,p2=3,p3=5,…. Find all positive integer n, for which p1!+p2!+⋯+pn!=ab, for some positive integer a,b>1, where k! denotes the product of all integers from 1 to k.
Solution
Directly checking yields: p1!=2,p1!+p2!=8=23,p1!+p2!+p3!=128=27 For n≥4, p1!+p2!+⋯+pn!=128+7!+11!+⋯+pn! is a number in which all summands are divisible by 25, except p4!=7!=24⋅315, which is divisible by 24 but not by 25. Therefore, this number cannot be a power of 2. If it is a power of another number, then since 24 divides it, it can only be 24 or x2. Let us show that it cannot be a square. Indeed, 2!+3!+⋯+pn! gives a remainder 2 modulo 3, which is impossible for squares.
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