Brahmagupta-Fibonacci Identity/Extension/Proof 1

Extension to Brahmagupta-Fibonacci Identity

Let $a_1, a_2, \ldots, a_n, b_1, b_2, \ldots, b_n$ be integers.

Then:

$\ds \prod_{j \mathop = 1}^n \paren { {a_j}^2 + {b_j}^2} = c^2 + d^2$

where $c, d \in \Z$.

That is: the product of any number of sums of two squares is also a sum of two squares.

Proof

From the extension to the general Brahmagupta-Fibonacci Identity:

$\ds \prod_{j \mathop = 1}^n \paren { {a_j}^2 + m {b_j}^2} = c^2 + m d^2$

for some $c, d \in \Z$, for all $m \in \Z$.

The result follows by setting $m = 1$.

$\blacksquare$