Find a six-digit number represented as a b c e d f that has the following property:
a b c d e f = ! a + ! b + ! c + ! d + ! e + ! f
The exclamation mark in front of the number indicates a sub-factorial which is defined as follows:
! n = n ! ( 1 − 1 ! 1 + 2 ! 1 − 3 ! 1 + ⋯ + ( − 1 ) n n ! 1 )
For examples: ! 2 = 1 and ! 6 = 2 6 5
The exclamation mark after the number denotes the factorial function ; for example 8 ! = 1 × 2 × 3 × ⋯ × 8 .
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If you examine the sub-factorials of 1 to 9 you will see that !9 = 133 496 and !8 = 14 833 and if you multiply both by 6 you get respectively 800 976 and 88 998. Here you can see that 9 must be one of the digits, and most likely 8 as well, and you start working from there.
This number that can be written as a function where you have all his digits participating, is called a narcissist number . Another example is 48 625 = 4 5 + 8 2 + 6 6 + 2 8 + 5 4 . See, only the digits of a narcissist number are there.
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148349 = !1 + !4 + !8 + !3 + !4 + !9