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# COMPLEX MULTIPLICATION

## §10. ELLIPTIC FUNCTIONS AND COMPLEX MULTIPLICATION

In Chapter Two we solved our problem of when a prime p can be written in the form x2 + ny2. The criterion from Theorem 9.2 states that, with finitely many exceptions, The key ingredient is the polynomial fn(x), which we know is the minimal polynomial of a primitive element of the ring class field of [ ]. But the proof of Theorem 9.2 doesn't explain how to find such a primitive element, so that currently we have only an abstract solution of the problem of p = x2 + ny2. In this chapter, we will use modular functions and the theory of complex multiplication to give a systematic method for finding fn(x).

In §10 we will study elliptic functions and introduce the idea of complex multiplication. A key role is played by the j-invariant of a lattice, and we will show that if is an order in an imaginary quadratic field K, then its j-invariant j( ) is an algebraic number. But before we can get to the real depth ...

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