Coideals of Hopf algebra coming from right (left) coideals K->K^+ - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-26T05:13:28Z http://mathoverflow.net/feeds/question/72359 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/72359/coideals-of-hopf-algebra-coming-from-right-left-coideals-k-k Coideals of Hopf algebra coming from right (left) coideals K->K^+ Marcin Szamotulski 2011-08-08T15:08:48Z 2011-08-08T16:10:54Z <p>If $H$ is a Hopf algebra over a field and $K$ is a a right or left coideal of $H$ then $K^+=K\cap\ker\epsilon$ is a coideal of $H$. Does this hold when $k$ is a commutative ring? If not what is a counter example.</p> <p>Thanks!</p>