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The Euclidean inner product is symmetric, linear in each argument, and positive-definite. Specifically, $\mathbf{a} \cdot \mathbf{a}$ equals the squared length of $\mathbf{a}$, which is positive for every nonzero vector $\mathbf{a}$.

Conditions: The vectors are real coordinate vectors.; The operation is the standard dot product.

The Euclidean inner product is symmetric, linear in each argument, and positive-definite. Specifically, $\mathbf{a} \cdot \mathbf{a}$ equals the squared length of $\mathbf{a}$, which is positive for every nonzero vector $\mathbf{a}$.

Conditions: The vectors are real coordinate vectors.; The operation is the standard dot product.