In number theory, a factorion in a given number base is a natural number that equals the sum of the factorials of its digits.[1][2][3] The name factorion was coined by the author Clifford A. Pickover.[4]
Let be a natural number. For a base , we define the sum of the factorials of the digits[5][6] of , , to be the following:
where is the number of digits in the number in base , is the factorial of and
is the value of the th digit of the number. A natural number is a -factorion if it is a fixed point for , i.e. if .[7] and are fixed points for all bases , and thus are trivial factorions for all , and all other factorions are nontrivial factorions.
For example, the number 145 in base is a factorion because .
For , the sum of the factorials of the digits is simply the number of digits in the base 2 representation since .
A natural number is a sociable factorion if it is a periodic point for , where for a positive integer , and forms a cycle of period . A factorion is a sociable factorion with , and a amicable factorion is a sociable factorion with .[8][9]
All natural numbers are preperiodic points for , regardless of the base. This is because all natural numbers of base with digits satisfy . However, when , then for , so any will satisfy until . There are finitely many natural numbers less than , so the number is guaranteed to reach a periodic point or a fixed point less than , making it a preperiodic point. For , the number of digits 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 .
The number of iterations needed for to reach a fixed point is the function's persistence of , and undefined if it never reaches a fixed point.
Let be a positive integer and the number base . Then:
Let the digits of be , and Then
Thus is a factorion for for all .
Let the digits of be , and . Then
Thus is a factorion for for all .
4 | 6 | 41 | 42 |
5 | 24 | 51 | 52 |
6 | 120 | 61 | 62 |
7 | 720 | 71 | 72 |
Let be a positive integer and the number base . Then:
Let the digits of be , and . Then
Thus is a factorion for for all .
3 | 4 | 13 |
4 | 21 | 14 |
5 | 116 | 15 |
6 | 715 | 16 |
All numbers are represented in base .
Base | Nontrivial factorion ( , )[10] | Cycles |
---|---|---|
2 | ||
3 | ||
4 | 13 | 3 → 12 → 3 |
5 | 144 | |
6 | 41, 42 | |
7 | 36 → 2055 → 465 → 2343 → 53 → 240 → 36 | |
8 |
3 → 6 → 1320 → 12 175 → 12051 → 175 | |
9 | 62558 | |
10 | 145, 40585 |
871 → 45361 → 871[9] 872 → 45362 → 872[8] |