Factorion

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A factorion is a natural number that equals the sum of the factorials of its decimal digits. For example, 145 is a factorion because 1! + 4! + 5! = 1 + 24 + 120 = 145.

There are just four factorions (in base 10) and they are 1, 2, 145 and 40585 (sequence A014080 in OEIS).

Upper bound[edit]

If n is a natural number of d digits that is a factorion, then 10d − 1n ≤ 9!d. This fails to hold for d ≥ 8 thus n has at most 7 digits, and the first upper bound is 9,999,999. But the maximum sum of factorials of digits for a 7 digit number is 9!*7 = 2,540,160 establishing the second upper bound. Going further, 9!6 is 2,177,280, and the only 7 digit number not larger than 2,540,160 containing six 9's is 1,999,999, which is not a factorion by inspection. The next highest sum would be given by 1,999,998, yielding a third upper bound of 1,854,721.

Other bases[edit]

If the definition is extended to include other bases, there are an infinite number of factorions. To see this, note that for any integer n > 3 the number n! + 1 is a factorion in base (n-1)!, in which it is denoted by the two digit string "n1". For example, 25 is a factorion in base 6, in which it is denoted by "41"; 121 is a factorion in base 24, in which it is denoted by "51".

For n > 2, n! + 1 is also a factorion in base n! − n + 1, in which it is denoted by the 2 digit string "1n". For example, 25 is a factorion in base 21, in which it is denoted by "14".

All positive integers are factorions in base 1, 1 is a factorion in every base, and 2 is a factorion in every base greater than 2.

The following tables lists all of the factorions in bases up to and including base 26.

Base n Factorion expressed

in base n

Factorion expressed

in base 10

1 1, 11, 111, etc. all numbers ≥1
≥1 1 1
>2 2 2
2 10 2
4 13 7
5 144 49
6 41 25
6 42 26
9 6 2558 41,282
10 145 145
10 4 0585 40,585
11 24 26
11 44 48
11 2 8453 40,472
13 8379 0C5B 519,326,767
14 8 B0DD 409C 12,973,363,226
15 661 1441
15 662 1442
16 260 F3B6 6BF9 2,615,428,934,649
17 8405 40,465
17 146F 2G85 00G4 43,153,254,185,213
17 146F 2G85 86G4 43,153,254,226,251
21 14 25
23 498J HHJI 5L7M 50F0 1,175,342,075,206,371,480,506
24 51 121
24 52 122
26 10 K2J3 82HG GF81 2,554,945,949,267,792,653
26 10 K2J3 82HG GF82 2,554,945,949,267,792,654

(sequence A193163 in OEIS)

References[edit]