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jmc

algebra senior

Problem

Let be defined by . How many complex numbers are there such that and both the real and the imaginary parts of are integers with absolute value at most ?
Solution
Suppose . We look for with such that are integers where .

First, use the quadratic formula:



Generally, consider the imaginary part of a radical of a complex number: , where .

.

Now let , then , , .

Note that if and only if . The latter is true only when we take the positive sign, and that ,

or , , or .

In other words, for all , satisfies , and there is one and only one that makes it true. Therefore we are just going to count the number of ordered pairs such that , are integers of magnitude no greater than , and that .

When , there is no restriction on so there are pairs;

when , there are pairs.

Thus there are numbers in total.
Final answer
399