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Here is the picture of RNN enter image description here

It's said, that hidden layer is fully connected(dash dots). But I don't understand why? For example I don't understand why the one from $a_3$ to $a_1$ exists. I thought information transfers from hidden state $a_{t-1}$ to $a_{t}$ and not other way around. Maybe I am confused by terminology.

Can you explain me?

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This is indeed an unconventional way to present the structure of an RNN, and I think your confusion is coming from conflating the time indices with the unit indices. Here's the "unrolled" network, with time indices in superscript:

enter image description here

In this diagram, each arrow represents a full set of connections from one layer to the other. For example, an arrow between two blue blocks actually stands in for 9 arrows between all the recurrent units of one layer to all the units of the next.

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  • $\begingroup$ thank you for your comment! So it means, for example, that units in layer 1 are fully connected with units in layer 2? $\endgroup$ – Daniel Yefimov Oct 14 at 18:23
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    $\begingroup$ If by "layer" you're referring to the time slices, then yes, the units at time [1] are fully connected to the units at time [2]. This corresponds to the dashed arrows in the figure in the original question. $\endgroup$ – tddevlin Oct 14 at 21:38

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