Start with the sequence {1, 2, 3, 4, 5, 6, 7, 8, ...}
Then change the sign of every other term so you'll have the first term positive, the second term negative, and so on.
That updates to {1, -2, 3, -4, 5, -6, 7, -8, ...}
Every odd term (1,3,5,..) is positive while every even term (-2,-4,-6) is negative.
