Big-Theta

Also known as: Big Theta, Θ notation

Big-Theta is a tight asymptotic bound: $f(n) = \Theta(g(n))$ means $f = O(g)$ and $f = \Omega(g)$. Growth matches $g$ up to constant factors.

Prefer $\Theta$ in exam writeups when both directions are clear: merge sort is $\Theta(n \log n)$, not merely $O(n \log n)$. Reserve plain Big-O when you only proved an upper bound (or when stating a loose ceiling on purpose).

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