# Every Tenth Power of Two Minus Every Third Power of Ten is Divisible By Three

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## Theorem

Let $x \in \Z_{\ge 0}$ be a non-negative integer.

Then $2^{10 x} - 10^{3 x}$ is divisible by $3$.

That is:

- $2^{10 x} - 10^{3 x} \equiv 0 \pmod 3$

## Proof

\(\ds 2^{10 x}\) | \(=\) | \(\ds \paren {2^{10} }^x\) | Power of Power | |||||||||||

\(\ds \) | \(=\) | \(\ds 1024^x\) | as $2^{10} = 1024$ | |||||||||||

\(\ds \) | \(=\) | \(\ds \paren {1000 + 24}^x\) | rewriting $1024$ as the sum of a power of $10$ and some integer | |||||||||||

\(\ds \) | \(=\) | \(\ds \sum_{k \mathop = 0}^n 1000^{x - k} \, 24^k\) | Binomial Theorem | |||||||||||

\(\text {(1)}: \quad\) | \(\ds \) | \(=\) | \(\ds 1000^x + \sum_{k \mathop = 1}^n 1000^{x - k} \, 24^k\) | extracting first term from summation | ||||||||||

\(\ds \) | \(=\) | \(\ds 1000^x + 24 \paren {\sum_{k \mathop = 1}^n 1000^{x - k} \, 24^{k - 1} }\) | extracting $24$ as a divisor | |||||||||||

\(\ds \leadsto \ \ \) | \(\ds 2^{10 x} - 1000^x\) | \(=\) | \(\ds 24 i\) | setting $i = \ds \sum_{k \mathop = 1}^n 1000^{x - k} \, 24^{k - 1}$ | ||||||||||

\(\ds \leadsto \ \ \) | \(\ds 2^{10 x} - 10^{3 x}\) | \(=\) | \(\ds 24 i\) | rewriting $1000^x$ as a power of $10$ | ||||||||||

\(\ds \) | \(=\) | \(\ds 3 k\) | setting $k = 8 i$ | |||||||||||

\(\ds \leadsto \ \ \) | \(\ds 2^{10x} - 10^{3 x}\) | \(\equiv\) | \(\ds 0 \pmod 3\) | Definition of Congruence Modulo Integer |

$\blacksquare$

## Examples

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Let us examine arguably the flagship example, the smallest number whose compliance is not obvious nor trivial:

- $1 \, 048 \, 576$

This is equal to $2^{20}$, which is equal to $2^{10 \times 2}$, thus one of the valid powers of $2$.

We then subtract $10^{3 \times 2}$, or $10^6$:

- $1 \, 048 \, 576 - 1 \, 000 \, 000 = 48 \, 576$

and we see that:

- $48 \, 576 = 16 \, 192 \times 3$

satisfying the theorem.