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Apr 26, 2019 at 8:49 comment added Ali Taghavi @KonstantinosKanakoglou Thanks for your suggestion. I read your comment very late(just now). i am sorry about that.
Feb 16, 2019 at 1:16 history edited Konstantinos Kanakoglou CC BY-SA 4.0
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Feb 2, 2019 at 2:02 comment added Konstantinos Kanakoglou @ user44191: Nice! and maybe it would be even better, if all these notes and comments, including the OP and the answer, were recorded in the form of a new question here (towards particular or general solutions of the OP functional equation) or maybe in a post in nLab.
Feb 1, 2019 at 3:00 comment added user44191 As a note, the example you have is in fact dual to a case of what @მამუკაჯიბლაძე wrote in the comments to the main post; the dual algebra to the group coalgebra is commutative, and the action you've chosen comes from multiplication by the idempotent $\mathbf{1}_S$ (which is central because the algebra is commutative).
Jan 31, 2019 at 22:10 history edited Konstantinos Kanakoglou CC BY-SA 4.0
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Jan 31, 2019 at 21:24 history edited Konstantinos Kanakoglou CC BY-SA 4.0
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Jan 31, 2019 at 21:15 history edited Konstantinos Kanakoglou CC BY-SA 4.0
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Jan 31, 2019 at 15:34 comment added user44191 I've made a chat:chat.stackexchange.com/rooms/89079/…
Jan 31, 2019 at 1:12 comment added Konstantinos Kanakoglou I am not sure why $T^{-1}(0)$ should be a coideal. Am i missing something simple ?
Jan 30, 2019 at 5:07 comment added user44191 And the statement that $T^*$ acts on $T^*(1)$ on $T^*(C^*)$ should be equivalent to a statement on the quotient coalgebra $C/T^{-1}$.
Jan 30, 2019 at 2:41 comment added user44191 There is no direct connection between $T(C)$ and $T^*(C^*)$; instead, $T^{-1}(0)$ is the subspace of $C$ that "identifies" $T^*(C^*)$, the subalgebra in the dual of $C$, and so is its natural counterpart. The right side of the resulting equation is then clear: $(\Delta \circ T^2)(T^{-1}(0)) = 0$, so $((T \otimes T) \circ \Delta)(T^{-1}(0)) = 0$. I think that should imply that $T^{-1}(0)$ is a coideal.
Jan 29, 2019 at 23:24 comment added Konstantinos Kanakoglou @user44191, my initial thought when i read your dual comment, was that $T(C)$ or $T^2(C)$ should be picked and that they would be coideals rather than subcoalgebras, in order to "read" the argument in the coalgebra language. However your proposal on picking $T^{-1}(0)$ seems interesting as well.
Jan 29, 2019 at 22:13 history edited Konstantinos Kanakoglou CC BY-SA 4.0
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Jan 29, 2019 at 22:10 comment added Konstantinos Kanakoglou In any case, this (i mean your dual statement on algebras) seems to be an interesting remark. Maybe it would be useful to discuss this and its possible translation, in a separate question (since this is not what the OP is asking about).
Jan 29, 2019 at 22:07 comment added Konstantinos Kanakoglou @user44191, thank you for your feedback. You are probably right in your observation that $T(C)$ need not be a subcoalgebra in general. So the first point of the "translation" is probably incorrect. I will remove it and try to come back after i will give it some more thought.
Jan 29, 2019 at 21:23 comment added user44191 I think instead of $T(C)$, $T^{-1}(0)$ is likely to be more useful.
Jan 29, 2019 at 21:02 comment added user44191 I think your translation is incorrect; removing the $T$ (analogous to where I essentially said $T(xy) \in C$), the inclusion goes in the other direction, that is, $(T \otimes T) \circ \Delta(C) = \Delta(T(T(C)) \subseteq \Delta(T(C))$.
Jan 29, 2019 at 20:25 history edited Konstantinos Kanakoglou CC BY-SA 4.0
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Jan 29, 2019 at 18:52 history edited Konstantinos Kanakoglou CC BY-SA 4.0
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Jan 29, 2019 at 5:57 history edited Konstantinos Kanakoglou CC BY-SA 4.0
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Jan 29, 2019 at 5:45 history edited Konstantinos Kanakoglou CC BY-SA 4.0
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Jan 29, 2019 at 5:12 history edited Konstantinos Kanakoglou CC BY-SA 4.0
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Jan 29, 2019 at 4:58 history edited Konstantinos Kanakoglou CC BY-SA 4.0
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Jan 28, 2019 at 23:48 history edited Konstantinos Kanakoglou CC BY-SA 4.0
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Jan 28, 2019 at 23:42 history edited Konstantinos Kanakoglou CC BY-SA 4.0
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Jan 28, 2019 at 2:42 history edited Konstantinos Kanakoglou CC BY-SA 4.0
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Jan 27, 2019 at 5:55 comment added Ali Taghavi Thank you very much for your answer.
Jan 26, 2019 at 18:23 history edited Konstantinos Kanakoglou CC BY-SA 4.0
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Jan 25, 2019 at 23:05 history edited Konstantinos Kanakoglou CC BY-SA 4.0
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Jan 25, 2019 at 22:58 history edited Konstantinos Kanakoglou CC BY-SA 4.0
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Jan 25, 2019 at 22:53 history answered Konstantinos Kanakoglou CC BY-SA 4.0