2006 AIME I Problems/Problem 13

Problem

For each even positive integer $x$, let $g(x)$ denote the greatest power of 2 that divides $x.$ For example, $g(20)=4$ and $g(16)=16.$ For each positive integer $n,$ let $S_n=\sum_{k=1}^{2^{n-1}}g(2k).$ Find the greatest integer $n$ less than 1000 such that $S_n$ is a perfect square.


Solution 1

Given $g : x \mapsto \max_{j : 2^j | x} 2^j$, consider $S_n = g(2) + \cdots + g(2^n)$. Define $S = \{2, 4, \ldots, 2^n\}$. There are $2^0$ elements of $S$ that are divisible by $2^n$, $2^1 - 2^0 = 2^0$ elements of $S$ that are divisible by $2^{n-1}$ but not by $2^n, \ldots,$ and $2^{n-1}-2^{n-2} = 2^{n-2}$ elements of $S$ that are divisible by $2^1$ but not by $2^2$.

Thus \begin{align*} S_n &= 2^0\cdot2^n + 2^0\cdot2^{n-1} + 2^1\cdot2^{n-2} + \cdots + 2^{n-2}\cdot2^1\\ &= 2^n + (n-1)2^{n-1}\\ &= 2^{n-1}(n+1).\end{align*} Let $2^k$ be the highest power of $2$ that divides $n+1$. Thus by the above formula, the highest power of $2$ that divides $S_n$ is $2^{k+n-1}$. For $S_n$ to be a perfect square, $k+n-1$ must be even. If $k$ is odd, then $n+1$ is even, hence $k+n-1$ is odd, and $S_n$ cannot be a perfect square. Hence $k$ must be even. In particular, as $n<1000$, we have five choices for $k$, namely $k=0,2,4,6,8$.

If $k=0$, then $n+1$ is odd, so $k+n-1$ is odd, hence the largest power of $2$ dividing $S_n$ has an odd exponent, so $S_n$ is not a perfect square.

In the other cases, note that $k+n-1$ is even, so the highest power of $2$ dividing $S_n$ will be a perfect square. In particular, $S_n$ will be a perfect square if and only if $(n+1)/2^{k}$ is an odd perfect square.

If $k=2$, then $n<1000$ implies that $\frac{n+1}{4} \le 250$, so we have $n+1 = 4, 4 \cdot 3^2, \ldots, 4 \cdot 13^2, 4\cdot 3^2 \cdot 5^2$.

If $k=4$, then $n<1000$ implies that $\frac{n+1}{16} \le 62$, so $n+1 = 16, 16 \cdot 3^2, 16 \cdot 5^2, 16 \cdot 7^2$.

If $k=6$, then $n<1000$ implies that $\frac{n+1}{64}\le 15$, so $n+1=64,64\cdot 3^2$.

If $k=8$, then $n<1000$ implies that $\frac{n+1}{256}\le 3$, so $n+1=256$.

Comparing the largest term in each case, we find that the maximum possible $n$ such that $S_n$ is a perfect square is $4\cdot 3^2 \cdot 5^2 - 1 = \boxed{899}$.

Solution 2

First note that $g(k)=1$ if $k$ is odd and $2g(k/2)$ if $k$ is even. so $S_n=\sum_{k=1}^{2^{n-1}}g(2k). = \sum_{k=1}^{2^{n-1}}2g(k) = 2\sum_{k=1}^{2^{n-1}}g(k) = 2\sum_{k=1}^{2^{n-2}} g(2k-1)+g(2k).$ $2k-1$ must be odd so this reduces to $2\sum_{k=1}^{2^{n-2}}1+g(2k) = 2(2^{n-2}+\sum_{k=1}^{2^n-2}g(2k)).$ Thus $S_n=2(2^{n-2}+S_{n-1})=2^{n-1}+2S_{n-1}.$ Further noting that $S_0=1$ we can see that $S_n=2^{n-1}\cdot (n-1)+2^n\cdot S_0=2^{n-1}\cdot (n-1)+2^{n-1}\cdot2=2^{n-1}\cdot (n+1).$ which is the same as above. To simplify the process of finding the largest square $S_n$ we can note that if $n-1$ is odd then $n+1$ must be exactly divisible by an odd power of $2$. However, this means $n+1$ is even but it cannot be. Thus $n-1$ is even and $n+1$ is a large even square. The largest even square $< 1000$ is $900$ so $n+1= 900 => n= \boxed{899}$

Solution 3 (Finding patterns and using recursion)

At first, this problem looks kind of daunting, but we can easily solve this problem by [u]finding patterns[/u] and [u]algebraic manipulations.[/u]

We first simplify all the messy notation in the $S_n$ term. Note that the problem asks us to find the smallest value of $n<1000$ such that there exists an integer $k$ that satisfies

\[k^2 = g(2) + g(4) + \cdots + g(2^n)\].

Since there is no obvious way to approach this problem, we start by experimenting with small values of $n$ to evaluate some $S_n$.

We play with these values:

\[S_1 = g(2) = 2\]

\[S_2 = g(2) + g(4) = 2+4 = 6\]

\[S_3 = g(2) + g(4) + g(6) + g(8) = 16\]

\[S_4 =  g(2) + g(4) + g(6) + g(8) + g(10) +g(12)+g(14)+g(16) = 40\]

We are certainly not going to expand all of this out... so let's look for patterns from these $4$ values!

Using a little bit of ingenuity, we note that

\[S_2 = 2+4 = S_1 + 4\]

\[S_3 = 2+4+2+8 = 8+8 = S_2 + S_1 + 8\]

\[S_4 = 2+4+2+8+2+4+2+16 = S_3 + S_2 + S_1 + 16\]

Aha! We see powers of two in each of our terms! Therefore, we can say that

\[S_2 = S_1 + 2^2\]

\[S_3 = S_2+S_1 + 2^3\]

\[S_4 = S_3 + S_2 + S_1 + 2^4\]

[b]We have a recursion![/b] Realistically, we would want to prove that the recursion works, but I currently don't know how to prove it. I will add a proof in the future when I have acquired enough number theory skills.

On the actual AIME, go with whatever patterns you see, because most likely those are the patterns that the test-takers want the students to see.

So we may generalize a formula for $S_n$:

\[S_n = 2^n + S_{n-1} + S_{n-2} + \cdots + S_2 + S_1\]

Uh oh... this formula is not in closed form. Looks like we'll have to use our recursion to develop one manually. We do so by using our recursion for $S_{n-1}$:

\[S_n = 2^n + (2^{n-1} + S_{n-2} + S_{n-3} + \cdots + S_2 + S_1) + S_{n-2} + S_{n-3} + \cdots + S_2 + S_1\]

\[S_n = 2^n + 2^{n-1} + 2 (S_{n-2} + S_{n-3} + \cdots + S_2 + S_1\]

Let's simplify a bit further, where we use our recursion for $S_{n-2}$.

\[S_n = 2^n + 2^{n-1} +(2S_{n-2}) + 2(S_{n-3} + S_{n-4} + \cdots + S_2 + S_1)\]

\[S_n = 2^n + 2^{n-1} + 2(2^{n-2}) + 2(S_{n-3} + S_{n-4} + \cdots + S_2 + S_1) + 2(S_{n-3} + S_{n-4} + \cdots + S_2 + S_1)\]

\[S_n = 2^n + 2^{n-1} + 2^{n-1} + 4(S_{n-3} + S_{n-4} + \cdots + S_2 + S_1)\]

We now see a pattern! Using the exact same logic, we can condense this whole messy thing into a closed form:

\[S_n = 2^n + \underbrace{2^{n-1}}_{n-2} + 2^{n-2}(S_1)\]

\[S_n = 2^n + 2^{n-1}(n-2) + 2^{n-1}\]

\[S_n = 2^n + 2^{n-1}(n-1)\]

\[S_n = 2^{n-1}(2 + (n-1)\]

\[S_n = 2^{n-1}(n+1)\]

We have our closed form, so now we can find the largest value of $n$ such that $S_n$ is a perfect square.

In order for $S_n$ to be a perfect square, we must have $n-1$ even and $n+1$ be a perfect square.

Since $n<1000$, we have $n+1 < 1001$. We first try $n+1 = 31^2 = 961$(since it is the smallest square below $1000$, which gives us $n=960$. But $n-1$ isn't even, so we discard this value.

Next, we try the second smallest value, which is $n = 30^2 = 900$, which tells us that $n=899$. $n-1$ is indeed even, and $n+1$ is a perfect square, so the largest value of $n$ such that $S_n$ is a perfect square is $899$.

Our answer is $\boxed{899}$.

-FIREDRAGONMATH16

See also

2006 AIME I (ProblemsAnswer KeyResources)
Preceded by
Problem 12
Followed by
Problem 14
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
All AIME Problems and Solutions

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