If , infinitely many primes satisfy
The th harmonic number is . Its asymptotic expansion begins
where is the Euler--Mascheroni constant.
Partial summation is the discrete analogue of integration by parts. If and is continuously differentiable, then
Sieve theory estimates the number of integers that avoid specified residue classes modulo primes.
For a finite integer set and a set of primes , the sifting function counts elements of divisible by no prime in below :
An upper-bound sieve bounds the size of a sifted set from above. In a dimension-one problem with one forbidden class modulo each relevant prime , its main density factor is comparable to .
If three nonproportional linear forms exclude three distinct residue classes modulo every sufficiently large prime, the upper-bound sieve has dimension three and gives an upper bound of order .
One form of Mertens' theorem is
Taking a ratio gives for .
A Dirichlet series is a series of the form
Multiplication of absolutely convergent Dirichlet series corresponds to Dirichlet convolution of their coefficients.
An Euler product factors a Dirichlet series into local factors indexed by prime numbers. For a multiplicative arithmetic function and in a half-plane of absolute convergence,
For every complex number with ,
Applying this to each prime-power term in logarithms of Euler products proves that
for and every completely multiplicative bounded by one.
Perron's formula recovers a summatory arithmetic function from its Dirichlet series by the inverse Mellin integral
with the usual convergence and endpoint conventions.
A truncated Perron integral over to recovers a summatory function with an error controlled by
for ; its Euler product is .
The completed zeta function
is an entire function satisfying . Thus every nontrivial zero is accompanied by , , and .
A nontrivial zero of the Riemann zeta function lies in the critical strip . The Functional equation of the Riemann zeta function reflects such zeros across the critical line .
The critical strip for the Riemann zeta function is .
The critical line is the symmetry line inside the critical strip.
The classical zero-free region asserts that for some ,
Together with bounds for the logarithmic derivative, it permits contour arguments with exponentially small errors in .
The logarithmic derivative of a nonzero differentiable function is . For , the Euler product for the Riemann zeta function gives
The classical zero-free region and a truncated Perron contour give
for some constant .
If, for some real ,
then partial summation continues meromorphically to with no pole except at . The zeta function has no zero to the right of the critical line, and its functional equation then implies the Riemann hypothesis.
For , the alternating Dirichlet series
converges locally uniformly and defines a holomorphic function. For ,
The identity continues the Riemann zeta function meromorphically to . Apparent singularities at nonreal zeros of are removable, as one sees by replacing with an integer for which . At , and , so has a simple pole of residue one.

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Analytic number theory is a branch of mathematics that uses tools and techniques from mathematical analysis to solve problems about integers, particularly concerning the distribution of prime numbers. It is a rich field that combines elements of number theory with methods from analysis, particularly infinite series, functions, and complex analysis.