Congruence Modulo 3 of Power of 2

Theorem
Let $n \in \Z_{\ge 0}$ be a positive integer.

Then:
 * $2^n \equiv \paren {-1}^n \pmod 3$

That is:
 * $\exists q \in \Z: 2^n = 3 q + \paren {-1}^n$

Proof
The proof proceeds by induction.

For all $n \in \Z_{\ge 0}$, let $\map P n$ be the proposition:
 * $2^n \equiv \paren {-1}^n \pmod 3$

$\map P 0$ is the case:

Thus $\map P 0$ is seen to hold.

Basis for the Induction
$\map P 1$ is the case:

Thus $\map P 1$ is seen to hold.

This is the basis for the induction.

Induction Hypothesis
Now it needs to be shown that, if $\map P k$ is true, where $k \ge 2$, then it logically follows that $\map P {k + 1}$ is true.

So this is the induction hypothesis:
 * $2^k \equiv \paren {-1}^k \pmod 3$

from which it is to be shown that:
 * $2^{k + 1} \equiv \paren {-1}^{k + 1} \pmod 3$

Induction Step
This is the induction step:

If $k$ is odd, this means:

If $k$ is even, this means:

So $\map P k \implies \map P {k + 1}$ and the result follows by the Principle of Mathematical Induction.

Therefore:
 * $\forall n \in \Z_{\ge 0}: 2^n \equiv \paren {-1}^n \pmod 3$

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