Thank you Laurent_R ! However the raw data does not exists as "chains". It is in a table where each row presents the number and the previous number. I just showed the chains in the commented lines to illustrate the problem. The chain building is very interesting itself however and can be useful in this project too. Cristoforo shows an example below. Thank you for the idea.

In reply to Re^2: Looking for the first item in the chain by vagabonding electron
in thread Looking for the first item in the chain by vagabonding electron

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