# Mertens' theorems

For Mertens' theorem on convergence of Cauchy products of series, see Cauchy_product#Convergence_and_Mertens.27_theorem.

In number theory, Mertens' theorems are three 1874 results related to the density of prime numbers proved by Franz Mertens.[1] "Mertens' theorem" may also refer to his theorem in analysis.

## In number theory

In the following, let $p\le n$ mean all primes not exceeding n.

Mertens' 1st theorem:

$\sum_{p \le n} \frac{\ln p}{p} - \ln n$

does not exceed 2 in absolute value for any $n\ge 2$.

Mertens' 2nd theorem:

$\lim_{n\to\infty}\left(\sum_{p\le n}\frac1p -\ln\ln n-M\right) =0,$

where M is the Meissel–Mertens constant. More precisely, Mertens[1] proves that the expression under the limit does not in absolute value exceed

$\frac 4{\ln(n+1)} +\frac 2{n\ln n}$

for any $n\ge 2$.

Mertens' 3rd theorem:

$\lim_{n\to\infty}\ln n\prod_{p\le n}\left(1-\frac1p\right)=e^{-\gamma},$

where γ is the Euler–Mascheroni constant.

### Changes in sign

In a paper [2] on the growth rate of the sum-of-divisors function published in 1983, Guy Robin proved that in Mertens' 2nd theorem the difference

$\sum_{p\le n}\frac1p -\ln\ln n-M$

changes sign infinitely often, and that in Mertens' 3rd theorem the difference

$\ln n\prod_{p\le n}\left(1-\frac1p\right)-e^{-\gamma}$

changes sign infinitely often. Robin's results are analogous to Littlewood's famous theorem that the difference π(x) − li(x) changes sign infinitely often. No analog of the Skewes number (an upper bound on the first natural number x for which π(x) > li(x)) is known in the case of Mertens' 2nd and 3rd theorems.

### Mertens' second theorem and the prime number theorem

Regarding this asymptotic formula Mertens refers in his paper to "two curious formula of Legendre",[1] the first one being Mertens' second theorem's prototype (and the second one being Mertens' third theorem's prototype: see the very first lines of the paper). He recalls that it is contained in Legendre's third edition of his "Théorie des nombres" (1830; it is in fact already mentioned in the second edition, 1808), and also that a more elaborate version was proved by Chebyshev in 1851.[3] Note that, already in 1737, Euler knew the asymptotic behaviour of this sum.[4]

Mertens diplomatically describes his proof as more precise and rigorous. In reality none of the previous proofs is acceptable by modern standards: Euler's computations involve the infinity (and the hyperbolic logarithm of infinity, and the logarithm of the logarithm of infinity!); Legendre's argument is heuristic; and Chebyshev's proof, although perfectly sound, makes use of the Legendre-Gauss conjecture, which will be proved in 1896 and then be better known as the prime number theorem.

Mertens' proof does not appeal to any unproved hypothesis (in 1874), and only to elementary real analysis. It comes 22 years before the first proof of the prime number theorem which, on the other hand, will heavily exploit the behaviour of the Riemann zeta function in the complex plane. Mertens' proof is in that respect remarkable. Indeed, with modern notation it yields

$\sum_{p\le x}\frac1p=\log\log x+M+O(1/\log x)$

whereas the prime number theorem (in its simplest form, without error estimate), can be shown to be equivalent to[5]

$\sum_{p\le x}\frac1p=\log\log x+M+o(1/\log x).$

## In summability theory

In summability theory, Mertens' theorem states that if a real or complex infinite series

$\sum_{n=1}^\infty a_n$

converges to A and another

$\sum_{n=1}^\infty b_n$

converges absolutely to B then their Cauchy product converges to AB.

## References

1. ^ a b c F. Mertens. J. reine angew. Math. 78 (1874), 46-62 Ein Beitrag zur analytischen Zahlentheorie
2. ^ Robin, G. (1983). "Sur l’ordre maximum de la fonction somme des diviseurs". Séminaire Delange–Pisot–Poitou, Théorie des nombres (1981–1982). Progress in Mathematics 38: 233–244.
3. ^ P.L. Tchebychev. Sur la fonction qui détermine la totalité des nombres premiers. Mémoires présentés à l'Académie Impériale des Sciences de St-Pétersbourg par divers savants, VI 1851, 141-157
4. ^ Leonhard Euler. Variae observationes circa series infinitas. Commentarii academiae scientiarum Petropolitanae 9 (1737), 160-188.
5. ^ Although this equivalence is not explicitly mentioned there,it can for instance be easily derived from the material in chapter I.3 of: G. Tenenbaum. Introduction to analytic and probabilistic number theory. Translated from the second French edition (1995) by C. B. Thomas. Cambridge Studies in Advanced Mathematics, 46. Cambridge University Press, Cambridge,1995.