Since all polynomials in the single variable \(n\) are of order \(O(n^k)\) for some \(k\), the classes known as polynomial time and polynomial space are respectively defined as \(\textbf{P} = \bigcup_{k \in \mathbb{N}} \textbf{TIME}(n^k)\) and \(\textbf{PSPACE} =  \bigcup_{k \in \mathbb{N}} \textbf{SPACE}(n^k)\). It is also standard to introduce the classes \(\textbf{EXP} =  \bigcup_{k \in \mathbb{N}} \textbf{TIME}(2^{n^k})\) (exponential time) and \(\textbf{L} = \textbf{SPACE}(\log(n))\) (logarithmic space).