We thought the game had finished. However...

When I captured a big group of the opponent, I get used to think I will win the game. Many players probably think so. But of course it's not always true. 

My 57th KGS game (vs [13k], 4 stones game, W)

The opponent resigned in this situation, because the M11 big black group was dead. I thought the decision was correct. However...

If the black had played to interrupt at E7, the white group marked triangle was dead. If so, the game came from behind.

I had not to play to brak an eye  at N8, but to play to connect at E7, because the M11 black group couldn't have two eyes, even if I played tenuki.

And I made a computer play as the white to check wｈether there was a sequence of moves with which the white group marked triangle could live or not.

The computer found a completely different play. 
If the white play to approach at S13, the black group marked triangle can not make two eyes!
I was not aware of it completely.

In the end, the decision of the opponent resigning was correct, because the play of the white to connect at E7 and to approach at S13 were miai. That is to say, the game had finished.

All moves are here.