where the x are all the same variable, the index is just to keep track of the number of ways to pair them. The first x can be paired with 2 n − 1 others, leaving 2 n − 2. The next unpaired x can be paired with 2 n − 3 different x leaving 2 n − 4, and so on. This means that Wick's theorem, uncorrected, says that the expectation value of x should be: