Could you explain me the change in the limits of this double sum? Is there any property about this change and how can I understand the correct change in a similar case?
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Denote by what is summed. We have to show that . We have and . Hence you only have to show that . To see that, we notice that the set in which and lie is a triangle.
Originally Posted by girdav ... we notice that the set in which and lie is a triangle. Could you explain me what you mean?
For the first sum, we sum for and if your draw these points on the upper half plane you will get a triangle.
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