Product is Sum, Sum is Product

i = 1 n a n = i = 1 n a n \displaystyle \sum_{i=1}^n a_{n} = \displaystyle \prod_{i=1}^{n} a_{n}

What is the maximum value of n n such that there exists a sequence of natural numbers a 1 , a 2 , a n a_{1}, a_{2}, \ldots a_{n} satisfying the equation above?

5 4 7 11 13 No such maximum exists

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1 solution

Louie Dy
Feb 6, 2016

Let ( a 1 , a 2 , a n ) (a_1, a_2, \ldots a_n) denote the solution sets for a value of n n .

If n = 1 n = 1 , then it is true for all a 1 = a 1 a_1 = a_1 .

If n = 2 n = 2 , then ( 2 , 2 ) (2, 2) satisfies the equation.

If n = 3 n = 3 , then ( 1 , 2 , 3 ) (1, 2, 3) satisfies the equation.

If n = 4 n = 4 , then ( 1 , 1 , 2 , 4 ) (1, 1, 2, 4) satisfies the equation.

If n = 5 n = 5 , then ( 1 , 1 , 1 , 2 , 5 ) (1, 1, 1, 2, 5) satisfies the equation.

As you increase n n , you'll notice that there is a pattern such that:

If n = k n = k for some k k , then we can just put ( k 2 ) (k-2) 1 1 's, a 2 2 and k k in the parentheses: ( 1 , 1 , 1 , 1 , 2 , k ) (1, 1, 1, \ldots 1, 2, k) .

Using this into the equation,

1 + 1 + 1 + + 1 + 2 + k = 1 × 1 × × 1 × 2 × k 1 + 1 + 1 + \ldots + 1 + 2 + k = 1 \times 1 \times \ldots \times 1 \times 2 \times k

k 2 + 2 + k = 1 k 2 × 2 × k k - 2 + 2 + k = 1^{k-2} \times 2 \times k

2 k = 2 k 2k = 2k

Thus, there is no maximum value.

Note that there may be additional sets that can satisfy a given value n n , but we just need a set to prove that a solution exists for that value.

Relevant sequence .

Pi Han Goh - 5 years, 4 months ago

What would be the answer if repeated numbers were not allowed?

Guilherme Pierina Marques - 5 years, 1 month ago

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Answer is 3. See my link above.

Pi Han Goh - 5 years, 1 month ago

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