The following is a list of the statements appearing in our posts about number theory.
Title and link  Statement 

Theorem N.1: Fundamental Theorem of Arithmetic (not proved)  Every integer number greater than 1 can be written as a product of prime numbers. Moreover such expression, called factorization or decomposition into prime factors, is unique apart from the order of the factors. 
Property N.1A: Within natural numbers, a divisor is less than or equal to a positive dividend  Let a and b be two natural numbers, with a \gt 0. If b \mid a, then b \leq a. 
Property N.1B: Prime number, characteristic property or alternative definition  A prime number is an integer number p > 1 such that, if p \mid bc, then p \mid b or p \mid c, for any integers b and c. In other terms, p cannot divide a product of integers bc without dividing at least one of the two factors. 
Theorem N.2: Infinity of primes  The number of primes is infinite. 
Property N.1: Overestimation of binomials  For all n > 0 and for all m: \binom{n}{m} \leq 2^{n1} 
Property N.2: Underestimation of the central binomials of the Pascal’s triangle  For all n \geq 2: \binom{n}{\frac{n}{2}} \geq \frac{2^n}{n} if n is even, and \binom{n}{\left \lceil \frac{n}{2} \right \rceil} = \binom{n}{\left \lfloor \frac{n}{2} \right \rfloor} \geq \frac{2^n}{n} if n is odd. 
Property N.3: Underestimation of the central binomial of Pascal’s triangle, even n  For all even n \geq 2: \binom{n}{\frac{n}{2}} \geq \sqrt{2^{n}} 
Proposition N.1: Majorization of the product of primes up to x  For all x > 0: \theta^{\star}(x) = \prod_{1 \leq p \leq x} p \leq 2^{2(x1)} 
Theorem N.3: Bertrand’s postulate  For all n > 0, there exists a prime number between n + 1 and 2n. 
Lemma N.1: Factorization of the binomial \binom{2n}{n}  In the factorization of the binomial \binom{2n}{n}, for n > 0, a prime number p cannot have an exponent greater than \log_p 2n. 
Proposition N.2: Computation of \psi^{\star}(x)  For every integer x > 0: \psi^{\star}(x) = \prod_{p \leq x} p^{\left \lfloor \log_p x \right \rfloor} 
Corollary of Proposition N.2: Overestimation of \psi^{\star}(x) with \pi(x)  For every integer x > 0: \psi^{\star}(x) \leq x^{\pi(x)} 
Proposition N.3: Underestimation of \psi^{\star}(x)  For every integer x > 0: \psi^{\star}(x) \geq \sqrt[3]{2^x} 
Lemma N.2: Reformulation of \psi^{\star}(x) (“calculation by rows”)  For every integer x > 0: \psi^{\star}(x) = \left(\prod_{p \leq x} p\right) \cdot \left(\prod_{p^2 \leq x} p\right) \cdot \dots \cdot \left(\prod_{p^R \leq x} p\right) 
Proposition N.4: Connection between \psi^{\star} and \theta^{\star} functions  For every integer x > 0: \psi^{\star}(x) = \theta^{\star}(x) \cdot \theta^{\star}(\sqrt{x}) \cdot \dots \cdot \theta^{\star}(\sqrt[R]{x}), where R := \left \lfloor \log_2 x \right \rfloor. 
Lemma N.3: Underestimation of \theta^{\star}(x) through \pi(x)  For every real number \delta \geq 0 and for every x \gt 1: \theta^{\star}(x) \gt \left(x^{\delta}\right)^{\pi(x)  x^{\delta}}. 
Theorem N.4: Asymptotic equivalence and order of magnitude of \theta(x) and \psi(x)  The functions \theta(x) and \psi(x) are asymptotically equivalent and have order x: \theta(x) \sim \psi(x), \theta(x) \asymp x \asymp \psi(x). 
Theorem N.5: Asymptotical equivalence between \pi(x) and \frac{\theta(x)}{\log x}  \pi(x) \sim \frac{\theta(x)}{\log x} 
Corollary of Theorem N.5: Asymptotical equivalence between \pi(x) and \frac{\psi(x)}{\log x}  \pi(x) \sim \frac{\psi(x)}{\log x} 
Corollary II of Theorem N.5: Chebyshev’s Theorem: order of magnitude of \pi(x)  \pi(x) \asymp \frac{x}{\log x} 
Lemma N.4: Lemma of bar chart area  Let c_1, c_2, \dots, c_n be nonnegative real numbers, with n > 0. Let f: \{1, 2, ..., n\} \rightarrow \mathbb{R} be a function. Then the area A of the bar chart made up of n rectangles, each having basis c_i and height f(i), given by A = c_1 f(1) + c_2 f(2) + \ldots + c_n f(n) = \sum_{i=1}^{n} c_i f(i), can also be computed with the formula \begin{aligned}A &= C_n f(n) + C_{n1} (f(n1)  f(n)) + \ldots + C_1 (f(1)  f(2)) \\&= \sum_{k = 1}^{n1} C_k (f(k)  f(k + 1)) + C_n f(n)\end{aligned} where C_k := c_1 + c_2 + \ldots + c_k = \sum_{i=1}^{k} c_i. 
Lemma N.5: Lemma of bar chart area, second form  Let c_1, c_2, \dots, c_n be nonnegative real numbers, with n > 0. Let f: \{1, 2, ..., n\} \rightarrow \mathbb{R} be a function, and \widetilde{f} a C^1 extension of f. Then the area A of the bar chart made up of n rectangles, each with basis c_i and height f(i), given by A = c_1 f(1) + c_2 f(2) + \ldots + c_n f(n) = \sum_{i=1}^{n} c_i f(i), can be also computed with the formula C(n) f(n)  \int_1^n \overline{C}(k) \widetilde{f}'(t) dt where C: \{1, 2, ..., n\} \rightarrow \mathbb{N} is the function defined by C(k) := c_1 + c_2 + \ldots + c_k = \sum_{i=1}^{k} c_i. 
Theorem N.6: Approximation of the sum of inverses of the first positive integers 
For all integers n \gt 0: \sum_{k = 1}^n \frac{1}{k} = 1 + \frac{1}{2} + \frac{1}{3} + \ldots + \frac{1}{n} \approx \log n + \gamma
where logarithm base is Napier’s number e, while \gamma \approx 0.58 is the Euler’s constant. In particular: \sum_{k = 1}^n \frac{1}{k} = 1 + \frac{1}{2} + \frac{1}{3} + \ldots + \frac{1}{n} = \log n + \gamma + O\left(\frac{1}{n}\right)

Property N.4: Upper and lower bounds for the value assumed by a function defined on integer numbers, by means of integrals of an extension 
Let f: I \rightarrow \mathbb{R} be a function defined on a set I \subset \mathbb{Z}.
Let \underset{\sim}{f}: \underline{I} \rightarrow \mathbb{R} be an extension of f, where \underline{I} := \bigcup_{n \in I} (n  1, n]. Then:

Lemma N.6: Order of magnitude of the sum of a fraction logarithms when the denominator changes  \sum_{n=1}^x \log \left(\frac{x}{n} \right) = O(x) 
Lemma N.6A: Order of magnitude of the sum of a fraction logarithms when the denominator changes, generalized form 
Let h \in \mathbb{R}, h \gt 0. Then: \sum_{n=1}^x \log \left(\frac{x}{n} \right) = O(x) 
Proposition N.5: Order of magnitude of the factorial function  x! = x^x \cdot e^{O(x)} 
Property N.5: Number of multiples of a given number, between 1 and x 
Let n and x be two positive integers. The number of multiples of n that belong to the range [1, x] is \left \lfloor \frac{x}{n} \right \rfloor. 
Proposition N.6: The factorial function and the \Lambda^{\star} function  For every integer x \gt 0: x! = \prod_{r = 1}^x \left( \Lambda^{\star}(r) \right)^{\left \lfloor \frac{x}{r} \right \rfloor} 
Property N.6: Connection between \Lambda^{\star} and \psi^{\star}  For every integer x \gt 0: \prod_{r = 1}^x \Lambda^{\star}(r) = \psi^{\star}(x) 
Proposition N.7: Order of magnitude of the factoriallike function based on \Lambda^{\star}  \prod_{r = 1}^x \left( \Lambda^{\star}(r) \right)^{\frac{x}{r}} = x^x \cdot e^{O(x)} 
Corollary of Proposition N.7: Order of magnitude of the factoriallike function based on \Lambda^{\star}  \sum_{r = 1}^x \frac{\Lambda(r)}{r} = \log x + O(1) 
Theorem N.7: Limit inferior and limit superior of the function \frac{\overline{\psi}(t)}{t}  \begin{cases} \lim \inf \frac{\overline{\psi}(t)}{t} \leq 1 \\ \lim \sup \frac{\overline{\psi}(t)}{t} \geq 1 \end{cases} 
Corollary of Theorem N.7: Limit inferior and limit superior of the function \overline{\pi}(t) / \frac{t}{\log t}  \begin{cases} \lim \inf \overline{\pi}(t) / \frac{t}{\log t} \leq 1 \\ \lim \sup \overline{\pi}(t) / \frac{t}{\log t} \geq 1 \end{cases} 
Corollary II of Theorem N.7: Chebyshev’s Theorem, strong version  If the function \pi(x) / \frac{x}{\log x} tends to a limit, that limit is 1. 
Theorem N.8: Prime number Theorem  \pi(x) \sim \frac{x}{\log x} 
Theorem N.9: Prime number Theorem, strong version  \pi(x) \sim \mathrm{Li}(x) 
Proposition N.7A: The functions W and V are limited  The functions W and V are limited. 
Lemma N.7: Sufficient condition for the validity of the Prime Number Theorem  The following is a sufficient condition for the validity of the Prime Number Theorem:
\limsup_{x \to +\infty} V(\log x) = 0
where the function V is defined in Definition N.13. 
Corollary I of Proposition N.7A: \alpha and \beta are real  The constants \alpha and \beta defined by Definition N.14 are real numbers. 
Lemma N.8  Let x \gt 0 be a fixed integer. The following inequalities are equivalent:

Theorem N.10: Selberg’s Theorem 
R(x) \log x + \sum_{n = 1}^x \Lambda(n) R\left(\frac{x}{n}\right) = O(x)

Property N.7: Characterization of squarefree integers  An integer number is squarefree if and only if in its factorization there are only distinct prime factors. 
Property N.8: A property about the count of divisors of an integer greater than 1  Let n \gt 1 an integer number. Let E_n and O_n be the following sets:
E_n := \left\{ \begin{gathered} \text{squarefree divisors of $n$,} \\ \text{which are a product of an even number of prime factors} \end{gathered}\right\}
O_n := \left\{ \begin{gathered} \text{squarefree divisors of $n$,} \\ \text{which are a product of an odd number of prime factors} \end{gathered}\right\}
Then E_n = O_n. 
Property N.9: A property about the product of divisors of a positive integer number with at least two distinct prime factors  Let n be a positive integer numbr with at least two distinct prime factors. Let E_n and O_n be the sets defined in Property N.8. Then \prod_{a \in E_n} a = \prod_{b \in O_n} b. 
Property N.9 (second form): A property about the product of divisors of a positive integer number with at least two distinct prime factors 
Let n be a positive integer number with at least two distinct prime factors. Then: \frac{\prod_{d \in P_n} d}{\prod_{d \in D_n} d} \cdot \prod_{d \in Q_n} d = \prod_{d \mid n} d^{\mu(d)} = 1

Property N.8 (second form): A property about the count of divisors of an integer greater than 1 
Let n \gt 1 be an integer number. Then: \sum_{d \mid n} \mu(d) = 0

Property N.10: A property about the product of divisors of a positive integer number 
Let n \in \mathbb{N}^{\star}. Then: \prod_{d \mid n} d^{\mu(d)} = \frac{1}{\Lambda^{\star}(n)}

Property N.10 (logarithmic form): A property about the product of divisors of a positive integer number 
Let n \in \mathbb{N}^{\star}. Then: \sum_{d \mid n} \mu(d) \log d =  \Lambda(n)

Property N.11: A property about the count of divisors of a positive integer number 
Let n \in \mathbb{N}^{\star}. Then: \sum_{d \mid n} \mu(d) = \begin{cases} 1 & \text{if $n = 1$} \\ 0 & \text{otherwise} \end{cases}

Property N.12: Expression of \Lambda^{\star} by means of \mu 
Let n \in \mathbb{N}^{\star}. Then: \prod_{d \mid n} \left( \frac{n}{d} \right)^{\mu(d)} = \Lambda^{\star}(n)

Property N.12 (logarithmic form): Expression of \Lambda by means of \mu 
Let n \in \mathbb{N}^{\star}. Then: \sum_{d \mid n} \mu(d) \log \frac{n}{d} = \Lambda(n)

Property N.13: Exchange between d and \frac{n}{d} in summations extended to the divisors of a positive integer 
Let n be a positive integer. Let f be any function defined on \mathbb{N}^{\star} \times \mathbb{N}^{\star} (the codomain does not matter^{(*)}). Then: \sum_{d \mid n} f\left(d, \frac{n}{d}\right) = \sum_{d \mid n} f\left(\frac{n}{d}, d\right)
\prod_{d \mid n} f\left(d, \frac{n}{d}\right) = \prod_{d \mid n} f\left(\frac{n}{d}, d\right)
(*) To be precise, it’s necessary that the operations of sum and product are defined on the codomain, and they must be commutative. However in this context there is no problem at all, because this is true for the numerical sets we are interested in, that are \mathbb{N}, \mathbb{Z} and \mathbb{R}. 
Property N.14: Equivalent form for summations with d and \frac{n}{d} 
Let n be a positive integer. Let f be any function defined on \mathbb{N}^{\star} \times \mathbb{N}^{\star} (the codomain does not matter^{(*)}). Then: \sum_{d \mid n} f\left(d, \frac{n}{d}\right) = \sum_{ab = n} f(a,b)
\prod_{d \mid n} f\left(d, \frac{n}{d}\right) = \prod_{ab = n} f(a,b)
(*) To be precise, it’s necessary that the operations of sum and product are defined on the codomain, and they must be commutative. However in this context there is no problem at all, because this is true for the numerical sets we are interested in, that are \mathbb{N}, \mathbb{Z} and \mathbb{R}. 
Property N.15: Third equivalent form for summations where the product of indexes divides a constant 
Let n be a positive integer. Let g and h be any two functions defined on \mathbb{N}^{\star} (the codomain does not matter^{(*)}). Then: \sum_{d \mid n} \sum_{d^{\prime} \mid d} g(d^{\prime}) h\left(\frac{d}{d^{\prime}}\right) = \sum_{ab \mid n} g(a) h(b)
\prod_{d \mid n} \prod_{d^{\prime} \mid d} g(d^{\prime}) h\left(\frac{d}{d^{\prime}}\right) = \prod_{ab \mid n} g(a) h(b)
(*) The same observations made about Property N.14 are also valid now 
Property N.16: Simplification, by means of the Möbius function, of a double summation extended to the divisors of a positive integer, and to the divisors of the divisors 
Let n be a positive integer. Let f be any function defined on \mathbb{N}^{\star} (the codomain does not matter^{(*)}). Then: \sum_{ab \mid n} \mu(a) f(b) = f(n)
(*) The same observations made about Property N.13 are also valid now 
Property N.17: Expression of logarithm as a summation of \Lambda 
Let n be a positive integer. Then: \sum_{d \mid n} \Lambda(d) = \log n

Property N.18: Möbius inversion formula 
Let n be a positive integer. Let f be any function defined on \mathbb{N}^{\star} (the codomain does not matter^{(*)}), and let g be the function defined for all n \in \mathbb{N}^{\star} as follows: g(n) := \sum_{d \mid n} \mu(d) f\left(\frac{n}{d}\right)
Then \sum_{d \mid n} g(d) = f(n)
for all n \in \mathbb{N}^{\star}. (*) The same observations made about Property N.13 are also valid now 
Lemma N.9: First lemma about summations with the Möbius function and the logarithm 
\sum_{n \leq x} \sum_{d \mid n} \mu(d) \log \left(\frac{x}{d}\right) = O(x)
where the function on the left is defined for x \in \mathbb{N}^{\star}. 
Lemma N.10: Second lemma about summations with the Möbius function and the logarithm 
\sum_{n \leq x} \sum_{d \mid n} \mu(d) \log^2 \left(\frac{x}{d}\right) = 2x \log x + O(x)
where the function on the left is defined for x \in \mathbb{N}^{\star}. 
Property N.19: Simplification of the double summation for n \leq x and for d \mid n 
Let x be a positive integer. Let f be any function defined on positive integer numbers (the codomain does not matter^{(*)}). Then: \sum_{n \leq x} \sum_{d \mid n} f(d) = \sum_{n \leq x} \left \lfloor \frac{x}{n} \right \rfloor f(n)
(*) To be precise, it’s necessary that the sum operation is defined on the codomain, and it must be commutative. However in this context there is no problem at all, because this is true for the numerical sets we are interested in (\mathbb{N}, \mathbb{Z} and \mathbb{R}). 
Property N.20: Second equivalent form for summations where the product of indexes is less than or equal to a constant 
Let x be a positive integer number. Let f be any function defined on \mathbb{N}^{\star} \times \mathbb{N}^{\star} (the codomain does not matter^{(*)}). Then: \sum_{n \leq x} \sum_{m \leq \left\lfloor \frac{x}{n} \right\rfloor} f(n, m) = \sum_{nm \leq x} f(n, m)
(*) The same observations made about Property N.14 are also valid now 
Lemma N.11: Third lemma about summations with the Möbius function and the logarithm 
Let x be a positive integer. Then: \sum_{nm \leq x} \Lambda(n) \Lambda(m)  \sum_{n \leq x} \Lambda(n) \log n = \sum_{n \leq x} \sum_{d \mid n} \mu(d) \log^2 d

Lemma N.12: Preliminary Lemma for Selberg’s Theorem 
Let x be a positive integer. Then: \sum_{nm \leq x} \Lambda(n) \Lambda(m) + \sum_{n \leq x} \Lambda(n) \log n = \sum_{n \leq x} \sum_{d \mid n} \mu(d) \log^2 \left( \frac{n}{d} \right)

Lemma N.13: Preliminary Lemma for Selberg’s Theorem: asymptotic estimate 
Let x be a positive integer. Then: \sum_{nm \leq x} \Lambda(n) \Lambda(m) + \sum_{n \leq x} \Lambda(n) \log n = 2x \log x + O(x)

Proposition N.8: Consequence of Selberg’s Theorem 
R(x) \log^2 x = O(x \log x) + \sum_{n \leq x} \left( \sum_{km = n} \Lambda(k) \Lambda(m)  \Lambda(n) \log n \right) R\left(\frac{x}{n} \right)

Corollary 1 of Proposition N.8: Consequence of Selberg’s Theorem 
R(x) \log^2 x \leq \sum_{n \leq x} \left(\sum_{km = n} \Lambda(k) \Lambda(m) + \Lambda(n) \log n\right) \leftR\left(\frac{x}{n}\right)\right + O(x)

Corollary 2 of Proposition N.8: Consequence of Selberg’s Theorem, second form 
For every positive integer x, there are some real numbers a_1, \ldots, a_x such that \sum_{n \leq x} a_n = 2x \log x + O(x) and R(x) \log^2 x \leq \sum_{n \leq x} a_n \leftR\left(\frac{x}{n}\right)\right + O(x)

Property N.21: Equivalent form for summations where the product of indexes is less than or equal to a constant 
Let n be a positive integer. Let f be any function defined on \mathbb{N}^{\star} \times \mathbb{N}^{\star} (the codomain does not matter^{(*)}). Then: \sum_{ab \leq n} f(a, b) = \sum_{k \leq n} \sum_{ab = k} f(a, b)
(*) The same observations made about Property N.14 are also valid now 
Property N.22: First equivalent form for summations where the product of indexes divides a constant 
Let n be a positive integer. Let f be any function defined on \mathbb{N}^{\star} \times \mathbb{N}^{\star} (the codomain does not matter^{(*)}). Then: \sum_{ad \mid n} f(a, d) = \sum_{ab = n} \sum_{d \mid b} f(a, d)
(*) The same observations made about Property N.14 are also valid now 
Property N.23: Second equivalent form for summations where the product of indexes divides a constant 
Let n be a positive integer. Let f be any function defined on \mathbb{N}^{\star} \times \mathbb{N}^{\star} (the codomain does not matter^{(*)}). Then: \sum_{ad \mid n} f(a, d) = \sum_{d \mid n} \sum_{a \mid \frac{n}{d}} f(a, d)
(*) The same observations made about Property N.14 are also valid now 
Proposition N.12: Consequence of Selberg’s Theorem, integral form 
R(x) \log^2 x \leq 2 \int_1^x \log t \left R\left(\frac{x}{t}\right) \right dt + O(x \log x)

Property N.24: Relationship between \alpha and \alpha^{\prime}, and between \beta and \beta^{\prime} 
With reference to Definition N.23 (Constants \alpha^{\prime} and \beta^{\prime}): \alpha \leq \alpha^{\prime}
\beta \leq \beta^{\prime}

Corollary II of Proposition N.7A: \alpha^{\prime} and \beta^{\prime} are real  The constants \alpha^{\prime} and \beta^{\prime} defined by Definition N.23 are real numbers. 
Proposition N.23: Relationship between \alpha and \beta^{\prime} 
With reference to Definitions N.23 and N.14: \alpha \leq \beta^{\prime}

Proposition N.24: Overestimation of the absolute value of the integral of V between 0 and a positive number 
There exists a real number A \gt 0 such that, for all \xi \gt 0: \left \int_0^{\log \xi} V(u)\ du \right \lt A

Corollary of Proposition N.24: Overestimation of the absolute value of the integral of V between two positive or null values 
There exists a real number A \gt 0 such that, for all a \geq 0 and for all b \gt a: \left \int_{a}^{b} V(u)\ du \right \lt 2A

Lemma N.14: Overestimation of the integral of V in the intervals in which it doesn’t change sign 
Let [a, b] be an interval in which the function V is always positive or always negative. Then there exists a positive constant A such that: \int_{a}^{b} V(u)\ du \lt 2A

Proposition N.25: Overestimation of the absolute value of V after one zero of it 
Let t be a zero of V. Then, for all h \geq 0: V(t + h) \leq 1  \frac{1}{e^h} + O\left(\frac{1}{t}\right)

Proposition N.26: Relationship between \alpha^{\prime} and \beta^{\prime} 
With reference to Definition N.23: \alpha^{\prime} \leq \beta^{\prime}

Lemma N.15: Overestimation of the integral of V better than the one by limit superior, first case 
Suppose that \alpha^{\prime} \gt 0. Let \delta and a be such that \delta \geq \alpha^{\prime} and a \geq 0. If in the interval [a, a + \delta  \alpha^{\prime}] the function V has at least one zero, and if there is a constant h \lt 1 such that: (\delta  \alpha^{\prime}) \alpha^{\prime} + \frac{(\alpha^{\prime})^2}{2} = \delta h \alpha^{\prime}
then: \int_a^{a+\delta} V(u)\ du \leq \delta h \alpha^{\prime} + o(1)

Lemma N.16: In an interval, if V has no zeroes it changes sign at most once 
Let I be a real interval in which V has no zeroes. Then V changes sign at most once in I. 
Lemma N.17: Overestimation of the integral of V better than the one by limit superior, second case 
Suppose that \alpha^{\prime} \gt 0. Let A be the constant of Lemma N.14, let \delta \geq \alpha^{\prime} and a \geq 0. If in the interval [a, a + \delta  \alpha^{\prime}] the function V has no zeroes, and if there is a constant k \lt 1 such that 4A + (\alpha^{\prime})^2 = \delta k \alpha^{\prime}
then: \int_a^{a+\delta} V(u)\ du \leq \delta k \alpha^{\prime} + o(1)

Proposition N.27: Overestimation of the integral of V better than the one by limit superior, in intervals of appropriate length 
Suppose that \alpha^{\prime} \gt 0. Let \delta \geq \frac{4A}{\alpha^{\prime}} + \frac{3 \alpha^{\prime}}{2}, where A is the constant of Lemma N.14. Then there exists a constant h \lt 1 such that, for all a \geq 0: \int_a^{a+\delta} V(u)\ du \leq \delta h \alpha^{\prime} + o(1)

Property N.25: Calculating the values of a multiplicative function 
Let f: \mathbb{N}^{\star} \rightarrow \mathbb{R} be a multiplicative function and let n be a positive integer. Let n = p_1^{a_1} \cdot \ldots \cdot p_k^{a_k} be the factorization of n (with p_1, \ldots, p_k distinct prime factors and a_1, \ldots, a_k positive integers). Then: f(n) = f\left(p_1^{a_1}\right) \cdot \ldots \cdot f\left(p_k^{a_k}\right)

Property N.26: Calculating the values of a completely multiplicative function 
Let f: \mathbb{N}^{\star} \rightarrow \mathbb{R} be a completely multiplicative function and let n be a positive integer. Let n = p_1^{a_1} \cdot \ldots \cdot p_k^{a_k} be the factorization of n (with p_1, \ldots, p_k distinct prime factors and a_1, \ldots, a_k positive integers). Then: f(n) = \left(f(p_1)\right)^{a_1} \cdot \ldots \cdot \left(f(p_k)\right)^{a_k}

Proposition N.28: Dual Möbius inversion formula 
Let n be a positive natural number and let f be a function defined on the divisors of n. g(d) := \sum_{d \mid d^{\prime} \mid n} f(d^{\prime})
Then, for every k \mid n: f(k) = \sum_{k \mid d \mid n} \mu \left(\frac{d}{k}\right) g(d)

Proposition N.29: Number of integers pairs having a fixed squarefree least common multiple 
Let d be a squarefree positive integer. The number of ordered pairs of positive integers having least common multiple d is equal to: 3^{\omega(d)}
