Multiplicative Identity for Quaternions
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Theorem
In the set of quaternions $\mathbb H$, the element:
- $\mathbf 1 + 0 \mathbf i + 0 \mathbf j + 0 \mathbf k$
serves as the identity element for quaternion multiplication.
This element is written $\mathbf 1$.
Proof
Let $\mathbf x = a \mathbf 1 + b \mathbf i + c \mathbf j + d \mathbf k$.
From the definition of quaternion multiplication:
\(\ds \mathbf x \mathbf 1\) | \(=\) | \(\ds \paren {a \cdot 1 - b \cdot 0 - c \cdot 0 - d \cdot 0} \mathbf 1\) | ||||||||||||
\(\ds \) | \(+\) | \(\ds \paren {a \cdot 0 + b \cdot 1 + c \cdot 0 - d \cdot 0} \mathbf i\) | ||||||||||||
\(\ds \) | \(+\) | \(\ds \paren {a \cdot 0 - b \cdot 0 + c \cdot 1 + d \cdot 0} \mathbf j\) | ||||||||||||
\(\ds \) | \(+\) | \(\ds \paren {a \cdot 0 + b \cdot 0 - c \cdot 0 + d \cdot 1} \mathbf k\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds a \mathbf 1 + b \mathbf i + c \mathbf j + d \mathbf k\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \mathbf x\) |
Similarly for $\mathbf 1 \mathbf x$.
$\blacksquare$