  A relationship between 4d gauge theory and 2d CFT will be reviewed from the
very basics. We will first cover the introductory material on the 2d CFT and on
the instantons of 4d gauge theory. Next we will explicitly calculate and check
the agreement of the norm of a coherent state on the 2d side and the instanton
partition function on the 4d side. We will then see how this agreement can be
understood from the perspective of string and M theory.
