# If $x^4 =x$ in a ring, then the ring is commutative. [duplicate]

Possible Duplicate:
Ring such that $x^4=x$ for all $x$

Let $R$ be a ring such that $a^4=a$ $,\forall a \in R$. How do I show that $R$ is commutative?

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## marked as duplicate by lhf, t.b., Martin Sleziak, Asaf Karagila, Gerry MyersonNov 16 '11 at 12:03

–  Hans Lundmark Nov 16 '11 at 9:27
I answered this here: math.stackexchange.com/questions/76792/… –  user641 Nov 16 '11 at 9:43

For arbitrary elements $a,b$ calculate $ab-ba = (ab-ba)^4 = (ba-ab)^4 = ba-ab$. So $2ab=2ba$ and hence $ab=ba$.
2 cannot be invertible, right? Otherwise, for any $a\in R$, $a=a^4=(-a)^4=-a$. So $2a=0$, which implies $a=0$. –  Paul Nov 16 '11 at 9:19