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Jul 15, 2021 at 15:48 comment added YCor @TarasBanakh ($e$ neutral element) If there is a 2-cycle, say $a\mapsto b\mapsto a$, so $a=bc$, $b=ad$, for some $c,d\in D$. Hence $a=bc=adc$, so $dc=1$. If $d\neq c$, $dc=e$ contradicts $D$ being non-product-one. So $d=c$, and hence $c^2=e$, and this is excluded since you checked $D$ has no element of order $2$.
Jul 4, 2021 at 14:41 history edited YCor CC BY-SA 4.0
simplified argument
Jul 4, 2021 at 9:26 history answered YCor CC BY-SA 4.0