Floor Function Returns

n 3 1 2 n 3 + 1 2 \left\lfloor \frac{n}{3}-\frac{1}{2}\right\rfloor\left\lfloor \frac{n}{3}+\frac{1}{2}\right\rfloor

Calculate the sum of squares of integers n n such that the expression is a prime number.

Image Credit: Wikimedia Floor Function


The answer is 139.

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1 solution

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Pranjal Jain
Mar 16, 2015

Let n 3 1 2 = A \frac{n}{3}-\frac{1}{2}=A

A A + 1 = A ( A + 1 ) \left\lfloor A\right\rfloor \left\lfloor A+1\right\rfloor\\=\left\lfloor A\right\rfloor \left( \left\lfloor A\right\rfloor+1\right )

Let A = k \left\lfloor A\right\rfloor=k

So, k ( k + 1 ) k(k+1) is a prime number where k is integer.

Possible values are 1 × 2 1×2 and 2 × 1 -2×-1

k = 1 , 2 A [ 2 , 1 ) [ 1 , 2 ) n 3 [ 1.5 , 0.5 ) [ 1.5 , 2.5 ) n [ 4.5 , 1.5 ) [ 4.5 , 7.5 ) k=1,-2\Rightarrow A\in [-2,-1) \cup [1,2)\\\Rightarrow \frac{n}{3}\in [-1.5,-0.5) \cup [1.5,2.5)\\\Rightarrow n\in [-4.5,-1.5)\cup [4.5,7.5)

So values of n n are { 4 , 3 , 2 , 5 , 6 , 7 } \{-4,-3,-2,5,6,7\}

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