You can find this in many scientific papers, common phrases indicating that the proper explanation is skipped, here sorted in the growing order of complexity:

• Rather trivial
• Then clearly
• Simple (small) algebraic transformation
• Tedious exercise leave up to the reader

Positional encoding

From: Attention is all you need this reprint with comments

where pos is the position and i is the dimension. That is, each dimension of the positional encoding corresponds to a sinusoid. The wavelengths form a geometric progression from $2\pi$ to $10000 \cdot 2\pi$. We chose this function because we hypothesized it would allow the model to easily learn to attend by relative positions, since for any fixed offset $k$, $PE_{pos+k}$ can be represented as a linear function of $PE_{pos}$. We chose the sinusoidal version because it may allow the model to extrapolate to sequence lengths longer than the ones encountered during training.

Jupyter notebook to illustrate it

What?

Maybe it's just me but I don't understand how sin/cos functions can save positional relations. I can understand convolutional operations how they transform data based on their positions.

Why $sin$ for even indices and $cos$ for odd ones?

Explanation

From: Neural machine translation with a Transformer

Since the model doesn't contain any recurrent or convolutional layers, it needs some way to identify word order, otherwise it would see the input sequence as a bag of words instance, how are you, how you are, you how are would be indistinguishable.

The position encoding function vibrates along the position axis at different frequencies depending on their location along the depth of the embedding vector.