Turning one of your opponents groups into a seki usually a big thing in a game, often leading to victory.
Are there ways to sekify a group?
Both should have no eye or the same size of eye. Taking outside liberties first.
This seki is a nice example of how you don’t actually have to own the shoulders of an eye. They just need to be denied to your opponent to make it a valid eye.
I kind of enjoy this thread so I’ll keep posting things that happened or could’ve happened when I come across them in my games 
This is from a DGS game I finished. I messed up trying to kill the Black group (which I think was possible) but basically ended up with the option to make a gote seki with L18.
I decided to play S2 instead as I thought it was bigger on points (~seki is about 7 points and the bottom right corner is sente ish and hard to guess how much it’s worth, could be 8-10 points.
Anyway I played s2 and Black played L18 to remove the seki and I got to capture the S3 stone instead.
(Edit: it might even be more than just the corner points - AI is seeing some idea to peep at n2 which could be bothersome depending on the moves in the corner)
N2 O2 L3… I do agree with AI 
From one of my games:
Actually Black’s move 97 should have been at B13. White 102 could have killed at A11.
I am Harry Fearnley, AKA hanezeki on OGS. Indeed, I did discover/invent this position while working on Problem 120 from Igo Hatsuyoron. At that time, I called it a Circular Hanezeki – now, I call it Circular Tail Seki. The position shown here is logically the simplest/smallest of such positions. I have recently created the largest of these, which required me to use a 19x19 board, and in which the mutual 3-stone captures are replaced by mutual 6-stone captures. I can also be contacted by email at harryfearnley@gmail.com
From a game today on 
server
There was a big gnarly fight and we both missed some big things here, but the seki occurred in the middle of the fighting which was novel for me, instead of at end game
High handicap game, where I’d lost anyway, but in theory Black can still make a seki in the top right if white doesn’t defend.
They don’t tend to happen to often but here’s a couple in two posts
This one is from a fork of a game where we went into a kind of capturing race variant which could’ve been a big ko or in this case wound up a big seki.
In the original game both groups connected out. White won both versions of the game after mistakes on both sides in the bottom left put white ahead early.
Number 2:
This was an over the board 2 stone handicap game where white thought it’d be a good idea not to invade the black territory until too late, and couldn’t find a way to kill the black stones on the bottom. Basically behind the whole game, and a chance to kill the black stones on the upper left turned into a seki.
This game just ended. It shows a quite big seki in top side.
Interestingly there could’ve been another seki, according to katago’s suggestions.
In last moves I (white) threatened a big group. My opponent resigned but katago says that everything could live: a piece with two eyes and another one in seki.
I like to come back to this thread every so often with another seki. From a recently finished correspondence game
White can and did make a seki but actually doing it turned out to be a 10 point mistake. Black could probably resist with a ko, but likely has much more to lose, so also not worth it.
Supposedly the better idea is for white to play L5, and just leave the possibility of a seki for later, since capturing 3 stones isn’t that big at the moment for black.
Forever ko seki.
Let see if I understand well.
Best that black can do is seki. (S3)
Is it necessary ?
White can try for a ko at best but there is no ko as black can connect (for a seki again)
Now black can get an even better result by fighting the ko and win it if ko threats are enough.
So conclusion is black can always make a seki and don’t have to add a stone. and depending of ko threats if white gets too greedy, even black seki can change into a territory.
That’s it ?
After White T1 Black can’t connect S1 because it makes a dead eye shape. Unless black has plenty of ko threats they should ignore, and afterwards winning the ko is not easy for white. They would need to capture with R1, ignore one threat and fill one of the liberties S3 or T3 (connecting with S1 by white is a seki), after which black re-captures the ko.
How did i forget that? So It’s seki because there are too many approach moves? And when becoming finally necessary, black can S3(or T3) to solidify ?