I’ve been trying to brush up on ring theory; it has been a long time since I studied rings in any depth and I need some ring theory to do some work in topology. In a previous post, I talked about ideal topologies and I might discuss divisor toplogies (starting with the ring of integers).
So, I grabbed an old text, skimmed the first part and came across an exercise:
an element is nilpotent if there is some positive integer such that . So, given nilpotent in a commutative ring one has to show that is also nilpotent and that this result might not hold if is not a commutative ring.
Examples: in the ring so is nilpotent. In the matrix ring of 2 by 2 matrices,
and are both nilpotent elements, though their sum:
is not; the square of this matrix is the identity matrix.
Immediately I thought to let be the smallest integers for and thought to apply the binomial theorem to (of course that is overkill; it is simpler to use . Lets use . I could easily see why but why were the middle terms also zero?
Then it dawned on me: for all . Duh. Now it made sense. 🙂