Factorion
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]
Definition
Let be a natural number. We define the sum of the factorial of the digits[5][6] of for base to be the following:
- .
where is the number of digits in the number in base , is the factorial of and
is the value of each digit of the number. A natural number is a -factorion if it is a fixed point for , which occurs if .[7] and are fixed points for all , 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 factorial of the digits is simply the number of digits in the base 2 representation.
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 a finite number of 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.
Factorions for
b = (k - 1)!
Let be a positive integer and the number base . Then:
- is a factorion for for all .
Let the digits of be , and . Then
Thus is a factorion for for all .
- 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 |
b = k! - k + 1
Let be a positive integer and the number base . Then:
- is a factorion for for all .
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 |
Programming example
The example below implements the sum of the factorial of the digits described in the definition above to search for factorions and cycles in Python.
def factorial(x: int) -> int:
total = 1
for i in range(0, x):
total = total * (i + 1)
return total
def sfd(x: int, b: int) -> int:
"""Sum of the factorial of the digits."""
total = 0
while x > 0:
total = total + factorial(x % b)
x = x // b
return total
def sfd_cycle(x: int, b: int) -> List[int]:
seen = []
while x not in seen:
seen.append(x)
x = sfd(x, b)
cycle = []
while x not in cycle:
cycle.append(x)
x = sfd(x, b)
return cycle
See also
References
- Sloane, Neil, "A014080", On-Line Encyclopedia of Integer Sequences
- Gardner, Martin (1978), "Factorial Oddities", Mathematical Magic Show: More Puzzles, Games, Diversions, Illusions and Other Mathematical Sleight-Of-Mind, Vintage Books, pp. 61 and 64, ISBN 9780394726236
- Madachy, Joseph S. (1979), Madachy's Mathematical Recreations, Dover Publications, p. 167, ISBN 9780486237626
- Pickover, Clifford A. (1995), "The Loneliness of the Factorions", Keys to Infinity, John Wiley & Sons, pp. 169–171 and 319–320, ISBN 9780471193340 – via Google Books
- Gupta, Shyam S. (2004), "Sum of the Factorials of the Digits of Integers", The Mathematical Gazette, The Mathematical Association, 88 (512): 258–261, doi:10.1017/S0025557200174996, JSTOR 3620841
- Sloane, Neil, "A061602", On-Line Encyclopedia of Integer Sequences
- Abbott, Steve (2004), "SFD Chains and Factorion Cycles", The Mathematical Gazette, The Mathematical Association, 88 (512): 261–263, doi:10.1017/S002555720017500X, JSTOR 3620842
- Sloane, Neil, "A214285", On-Line Encyclopedia of Integer Sequences
- Sloane, Neil, "A254499", On-Line Encyclopedia of Integer Sequences
- Sloane, Neil, "A193163", On-Line Encyclopedia of Integer Sequences