The critical line is the line of symmetry in the functional equation of the -function. In the usual normalization the functional equation associates to , so the critical line is .
In the usual normalization the Dirichlet series and the Euler product converge absolutely for The functional equation maps to . The remaining region, is known as the critical strip.
By the Euler product there are no zeros in , and by the functional equation there are only trivial zeros in . So all of the nontrivial zeros are in the critical strip, and the Riemann Hypothesis asserts that the nontrivial zeros are actually on the critical line.
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