Abstract: The Ising ferromagnet, consisting of magnetic spins, is
the simplest system showing phase transitions and critical phenomena
at finite temperatures. The Ising ferromagnet has played a central role
in our understanding of phase transitions and critical phenomena.
Also, the Ising ferromagnet explains the gas-liquid phase transitions
accurately. In particular, the Ising ferromagnet in a nonzero magnetic
field has been one of the most intriguing and outstanding unsolved
problems. We study analytically the partition function zeros in the
complex magnetic-field plane and the Yang-Lee edge singularity of
the infinite-range Ising ferromagnet in an external magnetic field.
In addition, we compare the Yang-Lee edge singularity of the
infinite-range Ising ferromagnet with that of the square-lattice Ising
ferromagnet in an external magnetic field.