# Prime omega function

In number theory, the prime omega functions $\omega (n)$ and $\Omega (n)$ count the number of prime factors of a natural number $n.$ The number of distinct prime factors is assigned to $\omega (n)$ (little omega), while $\Omega (n)$ (big omega) counts the total number of prime factors with multiplicity (see arithmetic function). That is, if we have a prime factorization of $n$ of the form $n=p_{1}^{\alpha _{1}}p_{2}^{\alpha _{2}}\cdots p_{k}^{\alpha _{k}}$ for distinct primes $p_{i}$ ($1\leq i\leq k$), then the prime omega functions are given by $\omega (n)=k$ and $\Omega (n)=\alpha _{1}+\alpha _{2}+\cdots +\alpha _{k}$. These prime-factor-counting functions have many important number theoretic relations.

## Properties and relations

The function $\omega (n)$ is additive and $\Omega (n)$ is completely additive. Little omega has the formula
$\omega (n)=\sum _{p\mid n}1,$
where notation p|n indicates that the sum is taken over all primes p that divide n, without multiplicity. For example, $\omega (12)=\omega (2^{2}3)=2$.
Big omega has the formulas
$\Omega (n)=\sum _{p^{\alpha }\mid n}1=\sum _{p^{\alpha }\parallel n}\alpha .$
The notation p$^α$|n indicates that the sum is taken over all prime powers p$^α$ that divide n, while p$^α$||n indicates that the sum is taken over all prime powers p$^α$ that divide n and such that n / p$^α$ is coprime to p$^α$. For example, $\Omega (12)=\Omega (2^{2}3^{1})=3$.
The omegas are related by the inequalities ω(n) ≤ Ω(n) and 2$^{ω(n)}$ ≤ d(n) ≤ 2$^{Ω(n)}$, where d(n) is the divisor-counting function. If Ω(n) = ω(n), then n is squarefree and related to the Möbius function by
    $\mu (n)=(-1)^{\omega (n)}=(-1)^{\Omega (n)}.$
If $\omega (n)=1$ then $n$ is a prime power, and if $\Omega (n)=1$ then $n$ is prime.
An asymptotic series for the average order of $\omega (n)$ is
    ${\frac {1}{n}}\sum \limits _{k=1}^{n}\omega (k)\sim \log \log n+B_{1}+\sum _{k\geq 1}\left(\sum _{j=0}^{k-1}{\frac {\gamma _{j}}{j!}}-1\right){\frac {(k-1)!}{(\log n)^{k}}},$
where $B_{1}\approx 0.26149721$ is the Mertens constant and $\gamma _{j}$ are the Stieltjes constants.
The function $\omega (n)$ is related to divisor sums over the Möbius function and the divisor function, including:
    $\sum _{d\mid n}|\mu (d)|=2^{\omega (n)}$ is the number of unitary divisors. OEIS: A034444
    $\sum _{d\mid n}|\mu (d)|k^{\omega (d)}=(k+1)^{\omega (n)}$
    $\sum _{r\mid n}2^{\omega (r)}=d(n^{2})$
    $\sum _{r\mid n}2^{\omega (r)}d\left({\frac {n}{r}}\right)=d^{2}(n)$
    $\sum _{d\mid n}(-1)^{\omega (d)}=\prod \limits _{p^{\alpha }||n}(1-\alpha )$
    $\sum _{\stackrel {1\leq k\leq m}{(k,m)=1}}\gcd(k^{2}-1,m_{1})\gcd(k^{2}-1,m_{2})=\varphi (n)\sum _{\stackrel {d_{1}\mid m_{1}}{d_{2}\mid m_{2}}}\varphi (\gcd(d_{1},d_{2}))2^{\omega (\operatorname {lcm} (d_{1},d_{2}))},\ m_{1},m_{2}{\text{ odd}},m=\operatorname {lcm} (m_{1},m_{2})$
    $\sum _{\stackrel {1\leq k\leq n}{\operatorname {gcd} (k,m)=1}}\!\!\!\!1=n{\frac {\varphi (m)}{m}}+O\left(2^{\omega (m)}\right)$
The characteristic function of the primes can be expressed by a convolution with the Möbius function:
    $\chi _{\mathbb {P} }(n)=(\mu \ast \omega )(n)=\sum _{d|n}\omega (d)\mu (n/d).$
A partition-related exact identity for $\omega (n)$ is given by
    $\omega (n)=\log _{2}\left[\sum _{k=1}^{n}\sum _{j=1}^{k}\left(\sum _{d\mid k}\sum _{i=1}^{d}p(d-ji)\right)s_{n,k}\cdot |\mu (j)|\right],$
where $p(n)$ is the partition function, $\mu (n)$ is the Möbius function, and the triangular sequence $s_{n,k}$ is expanded by
    $s_{n,k}=[q^{n}](q;q)_{\infty }{\frac {q^{k}}{1-q^{k}}}=s_{o}(n,k)-s_{e}(n,k),$
in terms of the infinite q-Pochhammer symbol and the restricted partition functions $s_{o/e}(n,k)$ which respectively denote the number of $k$'s in all partitions of $n$ into an odd (even) number of distinct parts.

## Continuation to the complex plane

A continuation of $\omega (n)$ has been found, though it is not analytic everywhere. Note that the normalized $\operatorname {sinc}$ function $\operatorname {sinc} (x)={\frac {\sin(\pi x)}{\pi x}}$ is used.
    $\omega (z)=\log _{2}\left(\sum _{n=1}^{\lceil Re(z)\rceil }\operatorname {sinc} \left(\prod _{m=1}^{\lceil Re(z)\rceil +1}\left(n^{2}+n-mz\right)\right)\right)$
This is closely related to the following partition identity. Consider partitions of the form
    $a={\frac {2}{c}}+{\frac {4}{c}}+\ldots +{\frac {2(b-1)}{c}}+{\frac {2b}{c}}$
where $a$, $b$, and $c$ are positive integers, and $a>b>c$. The number of partitions is then given by $2^{\omega (a)}-2$.

## Average order and summatory functions

An average order of both $\omega (n)$ and $\Omega (n)$ is $\log \log n$. When $n$ is prime a lower bound on the value of the function is $\omega (n)=1$. Similarly, if $n$ is primorial then the function is as large as
$\omega (n)\sim {\frac {\log n}{\log \log n}}$
on average order. When $n$ is a power of 2, then $\Omega (n)=\log _{2}(n).$
Asymptotics for the summatory functions over $\omega (n)$, $\Omega (n)$, and powers of $\omega (n)$ are respectively
    ${\begin{aligned}\sum _{n\leq x}\omega (n)&=x\log \log x+B_{1}x+o(x)\\\sum _{n\leq x}\Omega (n)&=x\log \log x+B_{2}x+o(x)\\\sum _{n\leq x}\omega (n)^{2}&=x(\log \log x)^{2}+O(x\log \log x)\\\sum _{n\leq x}\omega (n)^{k}&=x(\log \log x)^{k}+O(x(\log \log x)^{k-1}),k\in \mathbb {Z} ^{+},\end{aligned}}$
where $B_{1}\approx 0.2614972128$ is the Mertens constant and the constant $B_{2}$ is defined by
    $B_{2}=B_{1}+\sum _{p{\text{ prime}}}{\frac {1}{p(p-1)}}\approx 1.0345061758.$
The sum of number of unitary divisors is
$\sum _{n\leq x}2^{\omega (n)}=(x\log x)/\zeta (2)+O(x)$ (sequence A064608 in the OEIS)
Other sums relating the two variants of the prime omega functions include
    $\sum _{n\leq x}\left\{\Omega (n)-\omega (n)\right\}=O(x),$
and
    $\#\left\{n\leq x:\Omega (n)-\omega (n)>{\sqrt {\log \log x}}\right\}=O\left({\frac {x}{(\log \log x)^{1/2}}}\right).$

### Example I: A modified summatory function

In this example we suggest a variant of the summatory functions $S_{\omega }(x):=\sum _{n\leq x}\omega (n)$ estimated in the above results for sufficiently large $x$. We then prove an asymptotic formula for the growth of this modified summatory function derived from the asymptotic estimate of $S_{\omega }(x)$ provided in the formulas in the main subsection of this article above.
To be completely precise, let the odd-indexed summatory function be defined as
    $S_{\operatorname {odd} }(x):=\sum _{n\leq x}\omega (n)[n{\text{ odd}}],$
where $[\cdot ]$ denotes Iverson bracket. Then we have that
    $S_{\operatorname {odd} }(x)={\frac {x}{2}}\log \log x+{\frac {(2B_{1}-1)x}{4}}+\left\{{\frac {x}{4}}\right\}-\left[x\equiv 2,3{\bmod {4}}\right]+O\left({\frac {x}{\log x}}\right).$
The proof of this result follows by first observing that
    $\omega (2n)={\begin{cases}\omega (n)+1,&{\text{if }}n{\text{ is odd; }}\\\omega (n),&{\text{if }}n{\text{ is even,}}\end{cases}}$
and then applying the asymptotic result from Hardy and Wright for the summatory function over $\omega (n)$, denoted by $S_{\omega }(x):=\sum _{n\leq x}\omega (n)$, in the following form:
    ${\begin{aligned}S_{\omega }(x)&=S_{\operatorname {odd} }(x)+\sum _{n\leq \left\lfloor {\frac {x}{2}}\right\rfloor }\omega (2n)\\&=S_{\operatorname {odd} }(x)+\sum _{n\leq \left\lfloor {\frac {x}{4}}\right\rfloor }\left(\omega (4n)+\omega (4n+2)\right)\\&=S_{\operatorname {odd} }(x)+\sum _{n\leq \left\lfloor {\frac {x}{4}}\right\rfloor }\left(\omega (2n)+\omega (2n+1)+1\right)\\&=S_{\operatorname {odd} }(x)+S_{\omega }\left(\left\lfloor {\frac {x}{2}}\right\rfloor \right)+\left\lfloor {\frac {x}{4}}\right\rfloor .\end{aligned}}$

### Example II: Summatory functions for so-termed factorial moments of ω(n)

The computations expanded in Chapter 22.11 of Hardy and Wright provide asymptotic estimates for the summatory function
    $\omega (n)\left\{\omega (n)-1\right\},$
by estimating the product of these two component omega functions as
    $\omega (n)\left\{\omega (n)-1\right\}=\sum _{\stackrel {pq\mid n}{\stackrel {p\neq q}{p,q{\text{ prime}}}}}1=\sum _{\stackrel {pq\mid n}{p,q{\text{ prime}}}}1-\sum _{\stackrel {p^{2}\mid n}{p{\text{ prime}}}}1.$
We can similarly calculate asymptotic formulas more generally for the related summatory functions over so-termed factorial moments of the function $\omega (n)$.

## Dirichlet series

A known Dirichlet series involving $\omega (n)$ and the Riemann zeta function is given by
    $\sum _{n\geq 1}{\frac {2^{\omega (n)}}{n^{s}}}={\frac {\zeta ^{2}(s)}{\zeta (2s)}},\ \Re (s)>1.$
We can also see that
    $\sum _{n\geq 1}{\frac {z^{\omega (n)}}{n^{s}}}=\prod _{p}\left(1+{\frac {z}{p^{s}-1}}\right),|z|<2,\Re (s)>1,$
    $\sum _{n\geq 1}{\frac {z^{\Omega (n)}}{n^{s}}}=\prod _{p}\left(1-{\frac {z}{p^{s}}}\right)^{-1},|z|<2,\Re (s)>1,$
The function $\Omega (n)$ is completely additive, where $\omega (n)$ is strongly additive (additive). Now we can prove a short lemma in the following form which implies exact formulas for the expansions of the Dirichlet series over both $\omega (n)$ and $\Omega (n)$:
Lemma. Suppose that $f$ is a strongly additive arithmetic function defined such that its values at prime powers is given by $f(p^{\alpha }):=f_{0}(p,\alpha )$, i.e., $f(p_{1}^{\alpha _{1}}\cdots p_{k}^{\alpha _{k}})=f_{0}(p_{1},\alpha _{1})+\cdots +f_{0}(p_{k},\alpha _{k})$ for distinct primes $p_{i}$ and exponents $\alpha _{i}\geq 1$. The Dirichlet series of $f$ is expanded by
    $\sum _{n\geq 1}{\frac {f(n)}{n^{s}}}=\zeta (s)\times \sum _{p\mathrm {\ prime} }(1-p^{-s})\cdot \sum _{n\geq 1}f_{0}(p,n)p^{-ns},\Re (s)>\min(1,\sigma _{f}).$
Proof. We can see that
    $\sum _{n\geq 1}{\frac {u^{f(n)}}{n^{s}}}=\prod _{p\mathrm {\ prime} }\left(1+\sum _{n\geq 1}u^{f_{0}(p,n)}p^{-ns}\right).$
This implies that
    ${\begin{aligned}\sum _{n\geq 1}{\frac {f(n)}{n^{s}}}&={\frac {d}{du}}\left[\prod _{p\mathrm {\ prime} }\left(1+\sum _{n\geq 1}u^{f_{0}(p,n)}p^{-ns}\right)\right]{\Biggr |}_{u=1}=\prod _{p}\left(1+\sum _{n\geq 1}p^{-ns}\right)\times \sum _{p}{\frac {\sum _{n\geq 1}f_{0}(p,n)p^{-ns}}{1+\sum _{n\geq 1}p^{-ns}}}\\&=\zeta (s)\times \sum _{p\mathrm {\ prime} }(1-p^{-s})\cdot \sum _{n\geq 1}f_{0}(p,n)p^{-ns},\end{aligned}}$
wherever the corresponding series and products are convergent. In the last equation, we have used the Euler product representation of the Riemann zeta function.
The lemma implies that for $\Re (s)>1$,
    ${\begin{aligned}D_{\omega }(s)&:=\sum _{n\geq 1}{\frac {\omega (n)}{n^{s}}}=\zeta (s)P(s)\\&\ =\zeta (s)\times \sum _{n\geq 1}{\frac {\mu (n)}{n}}\log \zeta (ns)\\D_{\Omega }(s)&:=\sum _{n\geq 1}{\frac {\Omega (n)}{n^{s}}}=\zeta (s)\times \sum _{n\geq 1}P(ns)\\&\ =\zeta (s)\times \sum _{n\geq 1}{\frac {\phi (n)}{n}}\log \zeta (ns)\\D_{h}(s)&:=\sum _{n\geq 1}{\frac {h(n)}{n^{s}}}=\zeta (s)\log \zeta (s)\\&\ =\zeta (s)\times \sum _{n\geq 1}{\frac {\varepsilon (n)}{n}}\log \zeta (ns),\end{aligned}}$
where $P(s)$ is the prime zeta function, $h(n)=\sum _{p^{k}|n}{\frac {1}{k}}=\sum _{p^{k}||n}{H_{k}}$ where $H_{k}$ is the $k$-th harmonic number and $\varepsilon$ is the identity for the Dirichlet convolution, $\varepsilon (n)=\lfloor {\frac {1}{n}}\rfloor$.

## The distribution of the difference of prime omega functions

The distribution of the distinct integer values of the differences $\Omega (n)-\omega (n)$ is regular in comparison with the semi-random properties of the component functions. For $k\geq 0$, define
    $N_{k}(x):=\#(\{n\in \mathbb {Z} ^{+}:\Omega (n)-\omega (n)=k\}\cap [1,x]).$
These cardinalities have a corresponding sequence of limiting densities $d_{k}$ such that for $x\geq 2$
    $N_{k}(x)=d_{k}\cdot x+O\left(\left({\frac {3}{4}}\right)^{k}{\sqrt {x}}(\log x)^{\frac {4}{3}}\right).$
These densities are generated by the prime products
    $\sum _{k\geq 0}d_{k}\cdot z^{k}=\prod _{p}\left(1-{\frac {1}{p}}\right)\left(1+{\frac {1}{p-z}}\right).$
With the absolute constant ${\hat {c}}:={\frac {1}{4}}\times \prod _{p>2}\left(1-{\frac {1}{(p-1)^{2}}}\right)^{-1}$, the densities $d_{k}$ satisfy
    $d_{k}={\hat {c}}\cdot 2^{-k}+O(5^{-k}).$
Compare to the definition of the prime products defined in the last section of in relation to the Erdős–Kac theorem.