Sum of Sequence of Even Index Fibonacci Numbers

Theorem
Let $F_k$ be the $k$th Fibonacci number.

Then:
 * $\displaystyle \forall n \ge 1: \sum_{j \mathop = 1}^n F_{2 j} = F_{2 n + 1} - 1$

That is:
 * $F_2 + F_4 + F_6 + \cdots + F_{2 n} = F_{2 n + 1} - 1$

Proof
Proof by induction:

For all $n \in \N_{>0}$, let $P \left({n}\right)$ be the proposition:
 * $\displaystyle \sum_{j \mathop = 1}^n F_{2 j} = F_{2 n + 1} - 1$

Basis for the Induction
$P(1)$ is the case $F_2 = 1 = F_3 - 1$, which holds from the definition of Fibonacci numbers.

This is our basis for the induction.

Induction Hypothesis
Now we need to show that, if $P \left({k}\right)$ is true, where $k \ge 1$, then it logically follows that $P \left({k + 1}\right)$ is true.

So this is our induction hypothesis:
 * $\displaystyle \sum_{j \mathop = 1}^k F_{2 j} = F_{2 k + 1} - 1$

Then we need to show:
 * $\displaystyle \sum_{j \mathop = 1}^{k + 1} F_{2 j} = F_{2 k + 3} - 1$

Induction Step
This is our induction step:

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

Therefore:
 * $\displaystyle \forall n \ge 1: \sum_{j \mathop = 1}^n F_{2 j} = F_{2 n + 1} - 1$