rep. by pop. is a short form for representaion by population...
I was asking for an article that confirmed that Mararat had indeed been elected. We have that, he got in and that's good.
If that was what you were asking for, then it wasn't apparent from your post - neither in what you you said nor in its contextual positioning.
Oh - "representation by population" - you mean that the areas from which representatives are elected contain roughly equivalent numbers of voters instead of being based on areas without regard to whether or not there are roughly equivalent numbers of voters in the various areas.
That may well be so. But it does not distinguish the electoral system that it is used - whether it is voting for individuals or parties (or lists of candidates).
It is not the same thing as proportional representation - at all.
And, given that Marat was third on the party list for United Russia in the area in which he was standing in a system of proportional representation, there would have had to be a catastrophic falling off of votes for United Russia for him to have not got sufficient quotas to have been elected.
Once it was apparent that United Russia had got nearly 50% of the over-all vote and close on 45% of votes in the Nizhny Novgorod region (reported widely as preliminary results on 5 December 2011 ) then it was obvious that Marat had been elected.
Which is how come the ATP (Association of Tennis Professionals) Champions Tour could report that Marat had been elected (which you posted at #4591) prior to the Russian Central Election Commission formally announcing the final results.