P is also known to be at least as large as L, the class of problems decidable in a logarithmic amount of memory space. A decider using formula_8 space cannot use more than formula_9 time, because this is the total number of possible configurations; thus, L is a subset of P. Another important problem is whether L = P. We do know that P = AL, the set of problems solvable in logarithmic memory by alternating Turing machines. P is also known to be no larger than PSPACE, the class of problems decidable in polynomial space. Again, whether P = PSPACE is an open problem. To summarize: