If $H$ is a Hopf algebra, do we have $H^{cop}$ is a Hopf algebra? Let $H=(H, m, u, \Delta, \epsilon, S)$ be a Hopf algebra, see for example the lecture notes, where $m$ is the multiplication, $u$ is the unit, $\Delta$ is the comultiplication, $\epsilon$ is the counit, $S$ is the antipode. Let $H^{cop} = (H, m, u, \Delta^{op}, \epsilon, S)$, where $m,u,\epsilon,S$ are the same maps as the maps in $H$, and $\Delta^{op}=\tau \circ \Delta$, $\tau$ is the flip map. Is $H^{cop}$ a Hopf algebra?
I am asking this question because in the book: Hopf algebras and their actions on rings, pages 213, 214, it is said that $t_{M,N}$ is a braiding if $H^{cop}$ is a Hopf algebra. Why the condition is not "$H$ is a Hopf algebra" but "$H^{cop}$ is a Hopf algebra"? Are "$H$ is a Hopf algebra " and "$H^{cop}$ is a Hopf algebra" equivalent? Thank you very much.


 A: For a Hopf algebra $H$, $H^{cop}$ may or may not be a Hopf algebra. If it is, then the maps $t_{M,N}$ above define a braiding. 
A: The answer is generally no.
The conditions: "$H$ is a Hopf algebra" and "$H^{cop}$ is a Hopf algebra" are generally not equivalent. But they are, if we confine ourselves at the bialgebra level. To be more detailed: 
If $H=(H, m, u, \Delta, \epsilon)$ is a bialgebra, $H^{cop} = (H, m, u, \Delta^{op}, \epsilon)$ is also a bialgebra called co-opposite bialgebra. (and $H^{op} = (H, m^{op}, u, \Delta, \epsilon)$ is called the opposite bialgebra).
However, if $H=(H, m, u, \Delta, \epsilon, S)$ is a hopf algebra, $(H, m, u, \Delta^{op}, \epsilon, S)$ is not necessarily a hopf algebra. (Neither is $(H, m^{op}, u, \Delta, \epsilon, S)$).
A sufficient condition is the antipode to be invertible: If $H=(H, m, u, \Delta, \epsilon, S)$ is a hopf algebra with invertible (i.e.: bijective) antipode then $H^{cop} = (H, m, u, \Delta^{op}, \epsilon, S^{-1})$ is also a hopf algebra. (and $H^{op} = (H, m^{op}, u, \Delta, \epsilon, S^{-1})$ as well). 
These are now called the co-opposite and the opposite Hopf algebras respectively.
Now, if $S^2=Id$ thus $S=S^{-1}$, then the co-opposite $H^{cop} = (H, m, u, \Delta^{op}, \epsilon, S)$ and the opposi-te $H^{op} = (H, m^{op}, u, \Delta, \epsilon, S)$ are also hopf algebras. For example, this is the situation for either commutative or cocommutative hopf algebras.
(see also: https://math.stackexchange.com/a/1691054/195021 for a related discussion). 
