This line of thought leads to the question "Does sequence A"n" contain the number "n" ?" and the sequences , "Numbers "n" such that OEIS sequence A"n" contains "n"", and , ""n" is in this sequence if and only if "n" is not in sequence A"n"". Thus, the composite number 2808 is in A053873 because is the sequence of composite numbers, while the non-prime 40 is in A053169 because it's not in , the prime numbers. Each "n" is a member of exactly one of these two sequences, and in principle it can be determined "which" sequence each "n" belongs to, with two exceptions (related to the two sequences themselves):