Factorion

Let ${\displaystyle n}$ be a natural number. For a base ${\displaystyle b>1}$, we define the sum of the factorials of the digits^[5][6] of ${\displaystyle n}$, ${\displaystyle \operatorname {SFD} _{b}:\mathbb {N} \rightarrow \mathbb {N} }$, to be the following:

${\displaystyle \operatorname {SFD} _{b}(n)=\sum _{i=0}^{k-1}d_{i}!.}$

where ${\displaystyle k=\lfloor \log _{b}n\rfloor +1}$ is the number of digits in the number in base ${\displaystyle b}$, ${\displaystyle n!}$ is the factorial of ${\displaystyle n}$ and

${\displaystyle d_{i}={\frac {n{\bmod {b^{i+1}}}-n{\bmod {b^{i}}}}{b^{i}}}}$

is the value of the ${\displaystyle i}$th digit of the number. A natural number ${\displaystyle n}$ is a ${\displaystyle b}$-factorion if it is a fixed point for ${\displaystyle \operatorname {SFD} _{b}}$, i.e. if ${\displaystyle \operatorname {SFD} _{b}(n)=n}$.^[7] ${\displaystyle 1}$ and ${\displaystyle 2}$ are fixed points for all bases ${\displaystyle b}$, and thus are trivial factorions for all ${\displaystyle b}$, and all other factorions are nontrivial factorions.

For example, the number 145 in base ${\displaystyle b=10}$ is a factorion because ${\displaystyle 145=1!+4!+5!}$.

For ${\displaystyle b=2}$, the sum of the factorials of the digits is simply the number of digits ${\displaystyle k}$ in the base 2 representation since ${\displaystyle 0!=1!=1}$.

A natural number ${\displaystyle n}$ is a sociable factorion if it is a periodic point for ${\displaystyle \operatorname {SFD} _{b}}$, where ${\displaystyle \operatorname {SFD} _{b}^{c}(n)=n}$ for a positive integer ${\displaystyle c}$, and forms a cycle of period ${\displaystyle c}$. A factorion is a sociable factorion with ${\displaystyle c=1}$, and a amicable factorion is a sociable factorion with ${\displaystyle c=2}$.^[8][9]

All natural numbers ${\displaystyle n}$ are preperiodic points for ${\displaystyle \operatorname {SFD} _{b}}$, regardless of the base. This is because all natural numbers of base ${\displaystyle b}$ with ${\displaystyle k}$ digits satisfy ${\displaystyle b^{k-1}\leq n<b^{k}}$. Given that each of the ${\displaystyle k}$ digits is at most ${\displaystyle b-1}$, ${\displaystyle \operatorname {SFD} _{b}\leq (b-1)!k}$. However, when ${\displaystyle k\geq b}$, then ${\displaystyle b^{k-1}>(b-1)!(k)}$ for ${\displaystyle b>2}$, so any ${\displaystyle n}$ will satisfy ${\displaystyle n>\operatorname {SFD} _{b}(n)}$ until ${\displaystyle n<b^{b}}$. There are finitely many natural numbers less than ${\displaystyle b^{b}}$, so the number is guaranteed to reach a periodic point or a fixed point less than ${\displaystyle b^{b}}$, making it a preperiodic point. For ${\displaystyle b=2}$, the number of digits ${\displaystyle k\leq n}$ for any number, once again, making it a preperiodic point. This means also that there are a finite number of factorions and cycles for any given base ${\displaystyle b}$.

The number of iterations ${\displaystyle i}$ needed for ${\displaystyle \operatorname {SFD} _{b}^{i}(n)}$ to reach a fixed point is the ${\displaystyle \operatorname {SFD} _{b}}$ function's persistence of ${\displaystyle n}$, and undefined if it never reaches a fixed point.

b = (m − 1)!

Let ${\displaystyle m}$ be a positive integer and the number base ${\displaystyle b=(m-1)!}$. Then:

• ${\displaystyle n_{1}=mb+1}$ is a factorion for ${\displaystyle \operatorname {SFD} _{b}}$ for all ${\displaystyle m\geq 4}$.

Proof

Let the digits of ${\displaystyle n_{1}=d_{1}b+d_{0}}$ be ${\displaystyle d_{1}=m}$, and ${\displaystyle d_{0}=1.}$ Then

${\displaystyle \operatorname {SFD} _{b}(n_{1})=d_{1}!+d_{0}!}$

${\displaystyle =m!+1!}$

${\displaystyle =m(m-1)!+1}$

${\displaystyle =d_{1}b+d_{0}}$

${\displaystyle =n_{1}}$

Thus ${\displaystyle n_{1}}$ is a factorion for ${\displaystyle F_{b}}$ for all ${\displaystyle k}$.

• ${\displaystyle n_{2}=mb+2}$ is a factorion for ${\displaystyle \operatorname {SFD} _{b}}$ for all ${\displaystyle m\geq 4}$.

Proof

Let the digits of ${\displaystyle n_{2}=d_{1}b+d_{0}}$ be ${\displaystyle d_{1}=m}$, and ${\displaystyle d_{0}=2}$. Then

${\displaystyle \operatorname {SFD} _{b}(n_{2})=d_{1}!+d_{0}!}$

${\displaystyle =m!+2!}$

${\displaystyle =m(m-1)!+2}$

${\displaystyle =d_{1}b+d_{0}}$

${\displaystyle =n_{2}}$

Thus ${\displaystyle n_{2}}$ is a factorion for ${\displaystyle F_{b}}$ for all ${\displaystyle k}$.

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Factorions

${\displaystyle m}$	${\displaystyle b}$	${\displaystyle n_{1}}$	${\displaystyle n_{2}}$
4	6	41	42
5	24	51	52
6	120	61	62
7	720	71	72

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b = m! − m + 1

Let ${\displaystyle k}$ be a positive integer and the number base ${\displaystyle b=m!-m+1}$. Then:

• ${\displaystyle n_{1}=b+m}$ is a factorion for ${\displaystyle \operatorname {SFD} _{b}}$ for all ${\displaystyle m\geq 3}$.

Proof

Let the digits of ${\displaystyle n_{1}=d_{1}b+d_{0}}$ be ${\displaystyle d_{1}=1}$, and ${\displaystyle d_{0}=m}$. Then

${\displaystyle \operatorname {SFD} _{b}(n_{1})=d_{1}!+d_{0}!}$

${\displaystyle =1!+m!}$

${\displaystyle =m!+1-m+m}$

${\displaystyle =1(m!-m+1)+m}$

${\displaystyle =d_{1}b+d_{0}}$

${\displaystyle =n_{1}}$

Thus ${\displaystyle n_{1}}$ is a factorion for ${\displaystyle F_{b}}$ for all ${\displaystyle m}$.

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Factorions

${\displaystyle m}$	${\displaystyle b}$	${\displaystyle n_{1}}$
3	4	13
4	21	14
5	116	15
6	715	16

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Table of factorions and cycles of SFD_b

All numbers are represented in base ${\displaystyle b}$.

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Base ${\displaystyle b}$	Nontrivial factorion (${\displaystyle n\neq 1}$, ${\displaystyle n\neq 2}$)[10]	Cycles
2	${\displaystyle \varnothing }$	${\displaystyle \varnothing }$
3	${\displaystyle \varnothing }$	${\displaystyle \varnothing }$
4	13	3 → 12 → 3
5	144	${\displaystyle \varnothing }$
6	41, 42	${\displaystyle \varnothing }$
7	${\displaystyle \varnothing }$	36 → 2055 → 465 → 2343 → 53 → 240 → 36
8	${\displaystyle \varnothing }$	3 → 6 → 1320 → 12 175 → 12051 → 175
9	62558
10	145, 40585	871 → 45361 → 871[9] 872 → 45362 → 872[8]

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• Arithmetic dynamics
• Dudeney number
• Happy number
• Kaprekar's constant
• Kaprekar number
• Meertens number
• Narcissistic number
• Perfect digit-to-digit invariant
• Perfect digital invariant
• Sum-product number